feat(web): admin area with instance default theme, custom logo/favicon; .theme.json export

- Admin settings tab (admin-only) now holds user management plus a server-wide
  default theme editor (live preview + discard) and logo/favicon branding.
- New singleton InstanceSettings model + /api/instance router (public GET for
  the login screen; admin-gated theme PUT and logo/favicon upload/delete,
  reusing the avatar PIL/validation pattern).
- Instance default theme is the base users inherit and the target a per-colour
  "Reset" returns to (baseColor: instance default -> built-in).
- Custom favicon overrides the primary-colour tinting; custom logo replaces the
  top-left glyph+text, hard-capped so it only scales down and never breaks layout.
  Branding + defaults load before login (public endpoint).
- Theme export now uses the recognised .theme.json extension (old .theme still
  imports); import stays partial/unknown-key aware.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -17,7 +17,7 @@ STATIC_CACHE = f"public, max-age={STATIC_MAX_AGE_SECONDS}, must-revalidate"
 sys.path.insert(0, str(Path(__file__).parent))
 
 from database import Base, engine
-from routers import auth_router, birthdays_router, caldav_router, dav_router, google_router, groups_router, homeassistant_router, ical_router, local_router, profile_router, settings_router, users_router
+from routers import admin_router, auth_router, birthdays_router, caldav_router, dav_router, google_router, groups_router, homeassistant_router, ical_router, local_router, profile_router, settings_router, users_router
 
 logging.basicConfig(level=logging.INFO)
 
@@ -395,6 +395,7 @@ app.include_router(groups_router.router, prefix="/api/groups", tags=["groups"])
 app.include_router(ical_router.router, prefix="/api/ical", tags=["ical"])
 app.include_router(google_router.router, prefix="/api/google", tags=["google"])
 app.include_router(homeassistant_router.router, prefix="/api/homeassistant", tags=["homeassistant"])
+app.include_router(admin_router.router, prefix="/api/instance", tags=["instance"])
 # CalDAV publishing lives at root scope (no /api prefix) and must be registered
 # before the SPA catch-all so /dav/... isn't swallowed by the index fallback.
 app.include_router(dav_router.router, tags=["dav"])

backend/models.py

@@ -139,6 +139,20 @@ class UserSettings(Base):
     user = relationship("User", back_populates="settings")
 
 
+class InstanceSettings(Base):
+    """Server-wide (singleton, id=1) branding + default theme set by an admin.
+    Applies to everyone; a user's own settings still override the default theme."""
+    __tablename__ = "instance_settings"
+
+    id = Column(Integer, primary_key=True)  # always 1
+    # JSON {colorKey: "#RRGGBB"} — the instance default theme. Empty/NULL = use the
+    # client's built-in defaults. A user's own colour wins over this.
+    default_theme = Column(Text, nullable=True)
+    # Uploaded branding files (stored under DATA_DIR/branding). NULL = use bundled.
+    logo_filename = Column(String(255), nullable=True)
+    favicon_filename = Column(String(255), nullable=True)
+
+
 class AppPassword(Base):
     """Per-device app-specific password for CalDAV (Basic Auth).
 

backend/routers/admin_router.py

@@ -0,0 +1,194 @@
+"""Instance-wide (singleton) settings: an admin-defined default theme plus a
+custom logo and favicon. The GET endpoint is public (needed on the login screen,
+before auth); all writes require admin. Branding files are stored under
+DATA_DIR/branding and served via FileResponse, mirroring the avatar pattern."""
+
+import io
+import json
+import re
+from typing import Optional
+
+from fastapi import APIRouter, Depends, File, HTTPException, UploadFile
+from fastapi.responses import FileResponse
+from PIL import Image
+from pydantic import BaseModel
+from sqlalchemy.orm import Session
+
+import models
+from auth import get_current_admin
+from database import DATA_DIR, get_db
+
+router = APIRouter()
+
+BRANDING_DIR = DATA_DIR / "branding"
+BRANDING_DIR.mkdir(parents=True, exist_ok=True)
+MAX_BRANDING_SIZE = 5 * 1024 * 1024  # 5 MB
+ALLOWED_TYPES = {"image/jpeg", "image/png", "image/webp"}
+
+# Colour keys an admin may set as the instance default theme. Keep in sync with
+# the client's DEFAULT_COLORS / DEFAULT_SYNC colour keys (settings-sync.js).
+THEME_COLOR_KEYS = {
+    "primary_color", "accent_color", "today_color", "text_color", "bg_color",
+    "line_color", "surface_color", "month_divider_color", "month_label_color",
+    "hover_highlight_color", "icon_inactive_color", "icon_active_color",
+    "day_hover_color", "day_selected_color", "day_bg_color", "today_bg_color",
+}
+HEX_RE = re.compile(r"^#[0-9a-fA-F]{6}$")
+
+
+def _get_or_create(db: Session) -> models.InstanceSettings:
+    inst = db.query(models.InstanceSettings).filter(models.InstanceSettings.id == 1).first()
+    if not inst:
+        inst = models.InstanceSettings(id=1)
+        db.add(inst)
+        db.commit()
+        db.refresh(inst)
+    return inst
+
+
+def _mtime(filename: Optional[str]) -> int:
+    if not filename:
+        return 0
+    p = BRANDING_DIR / filename
+    try:
+        return int(p.stat().st_mtime)
+    except OSError:
+        return 0
+
+
+def _public_dict(inst: models.InstanceSettings) -> dict:
+    theme = {}
+    if inst.default_theme:
+        try:
+            theme = json.loads(inst.default_theme) or {}
+        except (ValueError, TypeError):
+            theme = {}
+    has_logo = bool(inst.logo_filename) and (BRANDING_DIR / (inst.logo_filename or "")).exists()
+    has_favicon = bool(inst.favicon_filename) and (BRANDING_DIR / (inst.favicon_filename or "")).exists()
+    return {
+        "default_theme": theme,
+        "has_logo": has_logo,
+        "has_favicon": has_favicon,
+        # Cache-busted URLs so a freshly uploaded asset is fetched immediately.
+        "logo_url": f"/api/instance/logo?v={_mtime(inst.logo_filename)}" if has_logo else None,
+        "favicon_url": f"/api/instance/favicon?v={_mtime(inst.favicon_filename)}" if has_favicon else None,
+    }
+
+
+# ── Public read ───────────────────────────────────────────
+@router.get("/")
+def get_instance(db: Session = Depends(get_db)):
+    return _public_dict(_get_or_create(db))
+
+
+@router.get("/logo")
+def get_logo(db: Session = Depends(get_db)):
+    inst = _get_or_create(db)
+    if not inst.logo_filename:
+        raise HTTPException(404, "No logo")
+    path = BRANDING_DIR / inst.logo_filename
+    if not path.exists():
+        raise HTTPException(404, "No logo")
+    return FileResponse(str(path), headers={"Cache-Control": "no-cache"})
+
+
+@router.get("/favicon")
+def get_favicon(db: Session = Depends(get_db)):
+    inst = _get_or_create(db)
+    if not inst.favicon_filename:
+        raise HTTPException(404, "No favicon")
+    path = BRANDING_DIR / inst.favicon_filename
+    if not path.exists():
+        raise HTTPException(404, "No favicon")
+    return FileResponse(str(path), headers={"Cache-Control": "no-cache"})
+
+
+# ── Admin writes ──────────────────────────────────────────
+class ThemeUpdate(BaseModel):
+    default_theme: dict  # {colorKey: "#RRGGBB"}; empty = reset to built-in
+
+
+@router.put("/theme")
+def set_default_theme(
+    data: ThemeUpdate,
+    db: Session = Depends(get_db),
+    admin: models.User = Depends(get_current_admin),
+):
+    # Keep only known colour keys with valid hex values.
+    clean = {
+        k: v.upper()
+        for k, v in (data.default_theme or {}).items()
+        if k in THEME_COLOR_KEYS and isinstance(v, str) and HEX_RE.match(v)
+    }
+    inst = _get_or_create(db)
+    inst.default_theme = json.dumps(clean) if clean else None
+    db.commit()
+    return {"ok": True, "default_theme": clean}
+
+
+async def _save_branding(file: UploadFile, kind: str) -> str:
+    """Validate + normalise an uploaded image and store it. Returns the filename."""
+    if file.content_type not in ALLOWED_TYPES:
+        raise HTTPException(400, "Only JPEG, PNG or WebP allowed")
+    raw = await file.read()
+    if len(raw) > MAX_BRANDING_SIZE:
+        raise HTTPException(400, "File too large (max 5 MB)")
+    try:
+        img = Image.open(io.BytesIO(raw)).convert("RGBA")
+    except Exception:
+        raise HTTPException(400, "Invalid image")
+    if kind == "favicon":
+        img = img.resize((128, 128), Image.LANCZOS)
+    else:  # logo: keep aspect ratio, cap the longest edge at 256px
+        img.thumbnail((256, 256), Image.LANCZOS)
+    filename = f"{kind}.png"
+    img.save(str(BRANDING_DIR / filename), "PNG")
+    return filename
+
+
+@router.post("/logo")
+async def upload_logo(
+    file: UploadFile = File(...),
+    db: Session = Depends(get_db),
+    admin: models.User = Depends(get_current_admin),
+):
+    inst = _get_or_create(db)
+    inst.logo_filename = await _save_branding(file, "logo")
+    db.commit()
+    return {"ok": True}
+
+
+@router.delete("/logo")
+def delete_logo(db: Session = Depends(get_db), admin: models.User = Depends(get_current_admin)):
+    inst = _get_or_create(db)
+    if inst.logo_filename:
+        p = BRANDING_DIR / inst.logo_filename
+        if p.exists():
+            p.unlink()
+        inst.logo_filename = None
+        db.commit()
+    return {"ok": True}
+
+
+@router.post("/favicon")
+async def upload_favicon(
+    file: UploadFile = File(...),
+    db: Session = Depends(get_db),
+    admin: models.User = Depends(get_current_admin),
+):
+    inst = _get_or_create(db)
+    inst.favicon_filename = await _save_branding(file, "favicon")
+    db.commit()
+    return {"ok": True}
+
+
+@router.delete("/favicon")
+def delete_favicon(db: Session = Depends(get_db), admin: models.User = Depends(get_current_admin)):
+    inst = _get_or_create(db)
+    if inst.favicon_filename:
+        p = BRANDING_DIR / inst.favicon_filename
+        if p.exists():
+            p.unlink()
+        inst.favicon_filename = None
+        db.commit()
+    return {"ok": True}

frontend/css/app.css

@@ -1390,6 +1390,26 @@ a { color: var(--primary); text-decoration: none; }
   font-size: 12px; font-weight: 700; text-decoration: none;
 }
 .theme-io-help:hover { background: var(--hover-highlight); color: var(--text-1); }
+
+/* Custom instance logo (replaces the glyph+text). Hard-capped so it can only
+   scale DOWN and never pushes the topbar/auth layout. */
+.topbar-logo-img { max-height: 40px; max-width: 150px; width: auto; height: auto; object-fit: contain; display: block; }
+.auth-logo-img   { max-height: 56px; max-width: 220px; width: auto; height: auto; object-fit: contain; display: block; }
+
+/* Admin panel: default-theme editor + branding */
+.admin-theme-grid { display: flex; flex-direction: column; gap: 8px; margin-top: 8px; }
+.admin-theme-row { display: grid; grid-template-columns: 1fr auto; align-items: center; gap: 12px; }
+.admin-theme-name { font-size: 13px; color: var(--text-2); }
+.admin-theme-actions { display: flex; gap: 8px; margin-top: 14px; }
+.admin-brand-row { display: flex; align-items: center; gap: 10px; margin-top: 10px; flex-wrap: wrap; }
+.admin-brand-label { width: 70px; font-size: 13px; color: var(--text-2); }
+.admin-brand-preview {
+  width: 48px; height: 48px; flex-shrink: 0;
+  display: flex; align-items: center; justify-content: center;
+  background: var(--bg-hover); border-radius: var(--radius-sm); overflow: hidden;
+}
+.admin-brand-preview img { max-width: 100%; max-height: 100%; object-fit: contain; }
+.admin-brand-none { font-size: 10px; color: var(--text-3); text-align: center; padding: 2px; }
 .settings-table-section {
   font-size: 12px; font-weight: 600; letter-spacing: .04em; text-transform: uppercase;
   color: var(--text-3); margin: 18px 0 6px; padding: 0 2px;

frontend/index.html

@@ -759,7 +759,7 @@
         <button class="settings-nav-btn active" data-panel="profile" data-i18n="settings_nav_profile">Profil</button>
         <button class="settings-nav-btn" data-panel="general" data-i18n="settings_nav_appearance">Darstellung</button>
         <button class="settings-nav-btn" data-panel="accounts" data-i18n="settings_nav_calendars">Kalender</button>
-        <button class="settings-nav-btn hidden" data-panel="users" id="settings-nav-users" data-i18n="settings_nav_users">Benutzerverwaltung</button>
+        <button class="settings-nav-btn hidden" data-panel="users" id="settings-nav-users" data-i18n="settings_nav_admin">Admin</button>
       </nav>
 
       <div class="settings-panels">
@@ -837,7 +837,7 @@
             <button type="button" class="btn btn-secondary btn-sm" id="cfg-theme-export" data-i18n="theme_export">Theme exportieren</button>
             <button type="button" class="btn btn-secondary btn-sm" id="cfg-theme-import" data-i18n="theme_import">Theme importieren</button>
             <a class="theme-io-help" id="cfg-theme-help" href="https://git.scarriffle.com/Scarriffle/Calendarr/src/branch/beta/THEME.md" target="_blank" rel="noopener noreferrer" data-i18n-title="theme_docs_hint" title="Erklärung der Theme-Parameter">?</a>
-            <input type="file" id="cfg-theme-file" accept=".theme,application/json,.json" hidden />
+            <input type="file" id="cfg-theme-file" accept=".theme.json,.json,.theme,application/json" hidden />
           </div>
         </div>
 
@@ -858,7 +858,7 @@
           <div id="birthday-settings"></div>
         </div>
 
-        <!-- Benutzerverwaltung -->
+        <!-- Admin: Benutzerverwaltung + Standard-Theme + Branding -->
         <div class="settings-panel" id="settings-panel-users">
           <h4 class="panel-title"><span data-i18n="settings_nav_users">Benutzerverwaltung</span> <span class="badge-admin">Admin</span></h4>
           <div id="users-list"></div>
@@ -877,6 +877,33 @@
             </label>
             <button class="btn btn-primary" id="new-user-save">Erstellen</button>
           </div>
+
+          <!-- Instanz-Standardtheme -->
+          <h4 class="panel-title" style="margin-top:28px" data-i18n="admin_default_theme">Standard-Theme</h4>
+          <p class="panel-desc" data-i18n="admin_default_theme_desc">Gilt für alle Nutzer, die keine eigene Farbe gesetzt haben. „Zurück" bei einer Farbe springt auf diesen Standard.</p>
+          <div id="admin-theme-grid" class="admin-theme-grid"></div>
+          <div class="admin-theme-actions">
+            <button class="btn btn-primary btn-sm" id="admin-theme-save" data-i18n="admin_theme_save">Standard speichern</button>
+            <button class="btn btn-ghost btn-sm" id="admin-theme-discard" data-i18n="admin_theme_discard">Verwerfen</button>
+          </div>
+
+          <!-- Branding: Logo & Favicon -->
+          <h4 class="panel-title" style="margin-top:28px" data-i18n="admin_branding">Branding</h4>
+          <p class="panel-desc" data-i18n="admin_branding_desc">Eigenes Logo (oben links) und Favicon (Tab-Symbol) für die ganze Instanz. PNG/JPEG/WebP, max. 5 MB.</p>
+          <div class="admin-brand-row">
+            <span class="admin-brand-label" data-i18n="admin_logo">Logo</span>
+            <div class="admin-brand-preview" id="admin-logo-preview"></div>
+            <button class="btn btn-secondary btn-sm" id="admin-logo-upload" data-i18n="admin_upload">Hochladen</button>
+            <button class="btn btn-ghost btn-sm" id="admin-logo-remove" data-i18n="admin_remove">Entfernen</button>
+            <input type="file" id="admin-logo-file" accept="image/png,image/jpeg,image/webp" hidden />
+          </div>
+          <div class="admin-brand-row">
+            <span class="admin-brand-label" data-i18n="admin_favicon">Favicon</span>
+            <div class="admin-brand-preview" id="admin-favicon-preview"></div>
+            <button class="btn btn-secondary btn-sm" id="admin-favicon-upload" data-i18n="admin_upload">Hochladen</button>
+            <button class="btn btn-ghost btn-sm" id="admin-favicon-remove" data-i18n="admin_remove">Entfernen</button>
+            <input type="file" id="admin-favicon-file" accept="image/png,image/jpeg,image/webp" hidden />
+          </div>
         </div>
 
       </div><!-- settings-panels -->

frontend/js/app.js

@@ -1,9 +1,13 @@
 import { api } from './api.js';
 import { initCalendar, showToast, openProfileModal } from './calendar.js';
 import { t } from './i18n.js';
+import { loadInstance } from './instance.js';
 
 // ── Bootstrap ─────────────────────────────────────────────
 async function boot() {
+  // Apply instance branding (logo/favicon/default theme) ASAP so the login and
+  // setup screens are already branded. Public endpoint — no token needed.
+  loadInstance();
   // Check if setup is required
   let setupRequired = false;
   try {

frontend/js/calendar.js

@@ -7,7 +7,8 @@ import { renderQuarter } from './views/quarter.js';
 import { openColorPicker } from './color-picker.js';
 import { openDatePicker, formatDtDisplay } from './date-picker.js';
 import { t, setLang, getLang } from './i18n.js';
-import { DEFAULT_COLORS, SETTING_GROUPS, SYNCABLE_KEYS, loadLocal, saveLocal, mergeEffective } from './settings-sync.js';
+import { DEFAULT_COLORS, SETTING_GROUPS, SYNCABLE_KEYS, loadLocal, saveLocal, mergeEffective, baseColor, setInstanceDefaults } from './settings-sync.js';
+import { loadInstance, reloadInstance, instanceConfig } from './instance.js';
 import { APP_VERSION } from './version.js';
 
 // Version im Impressum/Sidebar sichtbar, nicht im Tab-Titel.
@@ -123,6 +124,9 @@ export async function initCalendar() {
     api.get('/google/accounts').catch(() => []),
     api.get('/homeassistant/accounts').catch(() => []),
   ]);
+  // Ensure the instance default theme + branding are loaded before the first
+  // applyTheme(), so per-colour fallbacks resolve to the admin default.
+  await loadInstance();
 
   // Per-setting sync: the server's raw response is the shared source; effective
   // settings resolve each syncable key against the account-wide sync flags and
@@ -3359,7 +3363,7 @@ function populateSettings() {
   const user = JSON.parse(localStorage.getItem('user') || '{}');
   const usersNavBtn = document.getElementById('settings-nav-users');
   if (usersNavBtn) usersNavBtn.classList.toggle('hidden', !user.is_admin);
-  if (user.is_admin) loadUsers();
+  if (user.is_admin) { loadUsers(); loadAdminPanel(); }
 
   // Activate panel from URL or fall back to first visible
   const urlTab = readUrlState().stab;
@@ -3408,7 +3412,7 @@ function settingRowHtml(def) {
   } else if (def.type === 'toggle') {
     valueHtml = `<button type="button" class="sync-toggle ${val ? 'on' : ''}" data-vtoggle="${def.key}" role="switch" aria-checked="${!!val}"></button>`;
   } else if (def.type === 'color') {
-    const hex = normalizeHex(val, DEFAULT_COLORS[def.key]);
+    const hex = normalizeHex(val, baseColor(def.key));
     valueHtml = `<div class="settings-color-ctl">
       <input type="text" class="ev-color-hex" data-shex="${def.key}" maxlength="7" spellcheck="false" value="${hex}" />
       <div class="ev-color-preview" data-sprev="${def.key}" style="background:${hex}" title="${escHtml(t('color_pick'))}"></div>
@@ -3466,7 +3470,7 @@ function wireSettingRow(def) {
       applyTheme(state.settings);
     };
     prev.addEventListener('click', async () => {
-      const picked = await openColorPicker(prev, hex.value || DEFAULT_COLORS[def.key]);
+      const picked = await openColorPicker(prev, hex.value || baseColor(def.key));
       if (picked) { hex.value = picked.toUpperCase(); apply(picked); }
     });
     hex.addEventListener('input', () => {
@@ -3474,11 +3478,11 @@ function wireSettingRow(def) {
       if (norm) apply(norm);
     });
     hex.addEventListener('change', () => {
-      const norm = normalizeHex(hex.value, DEFAULT_COLORS[def.key]);
+      const norm = normalizeHex(hex.value, baseColor(def.key));
       hex.value = norm; apply(norm);
     });
     reset.addEventListener('click', () => {
-      const d = DEFAULT_COLORS[def.key];
+      const d = baseColor(def.key);
       hex.value = d; apply(d);
     });
   } else if (def.type === 'icon') {
@@ -3508,7 +3512,7 @@ function readAppearanceTable() {
       out[def.key] = el ? el.classList.contains('on') : !!state.settings[def.key];
     } else if (def.type === 'color') {
       const el = document.querySelector(`[data-shex="${def.key}"]`);
-      out[def.key] = normalizeHex(el && el.value, DEFAULT_COLORS[def.key]);
+      out[def.key] = normalizeHex(el && el.value, baseColor(def.key));
     } else if (def.type === 'icon') {
       const on = document.querySelector(`[data-sicon="${def.key}"] .group-icon-opt.on`);
       out[def.key] = on ? on.dataset.shareIcon : (state.settings[def.key] || 'share');
@@ -3563,7 +3567,7 @@ function exportTheme() {
   const url = URL.createObjectURL(blob);
   const a = document.createElement('a');
   a.href = url;
-  a.download = `${stamp}.theme`;
+  a.download = `${stamp}.theme.json`;
   document.body.appendChild(a);
   a.click();
   a.remove();
@@ -4436,11 +4440,152 @@ async function loadUsers() {
   } catch (e) { /* not admin */ }
 }
 
+// ── Admin: instance default theme + branding ──────────────
+let adminThemeDraft = {};      // {colorKey: hex} being edited (instance default)
+let adminPreviewActive = false; // whether the app view currently shows the draft
+
+// The colour rows offered in the default-theme editor (same keys as the user table).
+function adminColorDefs() {
+  return SETTING_GROUPS.flatMap(g => g.rows).filter(r => r.type === 'color');
+}
+
+// Restore the admin's own (personal) theme after a live preview.
+function restoreOwnTheme() {
+  adminPreviewActive = false;
+  applyTheme(state.settings);
+  applyFavicon(state.settings.primary_color);
+}
+
+function previewAdminTheme() {
+  adminPreviewActive = true;
+  applyTheme(adminThemeDraft);
+  applyFavicon(adminThemeDraft.primary_color);
+}
+
+function renderAdminThemeGrid() {
+  const grid = document.getElementById('admin-theme-grid');
+  if (!grid) return;
+  grid.innerHTML = adminColorDefs().map(def => {
+    const val = adminThemeDraft[def.key] || DEFAULT_COLORS[def.key];
+    return `<div class="admin-theme-row" data-arow="${def.key}">
+      <span class="admin-theme-name">${escHtml(t(def.labelKey))}</span>
+      <div class="ev-color-row">
+        <input type="text" class="ev-color-hex" data-ahex="${def.key}" maxlength="7" spellcheck="false" value="${val}" />
+        <div class="ev-color-preview" data-aprev="${def.key}" style="background:${val}" title="${escHtml(t('color_pick'))}"></div>
+        <button type="button" class="icon-btn ev-color-reset" data-areset="${def.key}" title="${escHtml(t('reset'))}">${RESET_ICON_SVG}</button>
+      </div>
+    </div>`;
+  }).join('');
+
+  adminColorDefs().forEach(def => {
+    const row = grid.querySelector(`.admin-theme-row[data-arow="${def.key}"]`);
+    if (!row) return;
+    const hex = row.querySelector('[data-ahex]');
+    const prev = row.querySelector('[data-aprev]');
+    const reset = row.querySelector('[data-areset]');
+    const apply = (color) => {
+      adminThemeDraft[def.key] = color;
+      prev.style.background = color;
+      previewAdminTheme();
+    };
+    prev.addEventListener('click', async () => {
+      const picked = await openColorPicker(prev, hex.value || DEFAULT_COLORS[def.key]);
+      if (picked) { hex.value = picked.toUpperCase(); apply(picked); }
+    });
+    hex.addEventListener('input', () => { const n = normalizeHex(hex.value, null); if (n) apply(n); });
+    hex.addEventListener('change', () => { const n = normalizeHex(hex.value, DEFAULT_COLORS[def.key]); hex.value = n; apply(n); });
+    reset.addEventListener('click', () => {
+      // Reset a default-theme colour to the built-in default.
+      const d = DEFAULT_COLORS[def.key];
+      hex.value = d; apply(d);
+    });
+  });
+}
+
+function renderAdminBranding() {
+  const logoPrev = document.getElementById('admin-logo-preview');
+  const favPrev = document.getElementById('admin-favicon-preview');
+  const ph = `<span class="admin-brand-none">${escHtml(t('admin_brand_default'))}</span>`;
+  if (logoPrev) logoPrev.innerHTML = instanceConfig.has_logo ? `<img src="${instanceConfig.logo_url}" alt="">` : ph;
+  if (favPrev) favPrev.innerHTML = instanceConfig.has_favicon ? `<img src="${instanceConfig.favicon_url}" alt="">` : ph;
+  const logoRemove = document.getElementById('admin-logo-remove');
+  const favRemove = document.getElementById('admin-favicon-remove');
+  if (logoRemove) logoRemove.classList.toggle('hidden', !instanceConfig.has_logo);
+  if (favRemove) favRemove.classList.toggle('hidden', !instanceConfig.has_favicon);
+}
+
+// Populate the admin panel (default-theme editor + branding). Admin-only.
+function loadAdminPanel() {
+  adminThemeDraft = { ...(instanceConfig.default_theme || {}) };
+  renderAdminThemeGrid();
+  renderAdminBranding();
+}
+
+async function uploadBranding(kind, input) {
+  const file = input.files && input.files[0];
+  input.value = '';
+  if (!file) return;
+  try {
+    const form = new FormData();
+    form.append('file', file);
+    await api.upload(`/instance/${kind}`, form);
+    await reloadInstance();          // re-fetch + apply logo/favicon live
+    renderAdminBranding();
+    showToast(t('admin_branding_saved'));
+  } catch (e) { showToast(e.message, true); }
+}
+
+async function removeBranding(kind) {
+  try {
+    await api.delete(`/instance/${kind}`);
+    await reloadInstance();
+    renderAdminBranding();
+    showToast(t('admin_branding_removed'));
+  } catch (e) { showToast(e.message, true); }
+}
+
+function bindAdminPanel() {
+  const save = document.getElementById('admin-theme-save');
+  if (save) save.addEventListener('click', async () => {
+    try {
+      await api.put('/instance/theme', { default_theme: adminThemeDraft });
+      setInstanceDefaults(adminThemeDraft);
+      instanceConfig.default_theme = { ...adminThemeDraft };
+      restoreOwnTheme();             // back to the admin's own view
+      showToast(t('admin_theme_saved'));
+    } catch (e) { showToast(e.message, true); }
+  });
+  const discard = document.getElementById('admin-theme-discard');
+  if (discard) discard.addEventListener('click', () => {
+    adminThemeDraft = { ...(instanceConfig.default_theme || {}) };
+    renderAdminThemeGrid();
+    restoreOwnTheme();
+  });
+
+  const bindUpload = (btnId, fileId, kind) => {
+    const btn = document.getElementById(btnId);
+    const file = document.getElementById(fileId);
+    if (btn && file) {
+      btn.addEventListener('click', () => file.click());
+      file.addEventListener('change', () => uploadBranding(kind, file));
+    }
+  };
+  bindUpload('admin-logo-upload', 'admin-logo-file', 'logo');
+  bindUpload('admin-favicon-upload', 'admin-favicon-file', 'favicon');
+  const logoRemove = document.getElementById('admin-logo-remove');
+  if (logoRemove) logoRemove.addEventListener('click', () => removeBranding('logo'));
+  const favRemove = document.getElementById('admin-favicon-remove');
+  if (favRemove) favRemove.addEventListener('click', () => removeBranding('favicon'));
+}
+
 function bindSettingsModal() {
   // Global "sync everything" master toggle (per-row toggles are wired per render).
   const syncAllBtn = document.getElementById('cfg-sync-all');
   if (syncAllBtn) syncAllBtn.addEventListener('click', toggleSyncAll);
 
+  // Admin panel (default-theme editor + branding) — buttons bound once.
+  bindAdminPanel();
+
   // Theme export / import (client-side .theme files).
   const themeExportBtn = document.getElementById('cfg-theme-export');
   if (themeExportBtn) themeExportBtn.addEventListener('click', exportTheme);
@@ -4902,6 +5047,8 @@ function closeModal(id) {
   document.getElementById(id).classList.add('hidden');
   if (id === 'modal-settings') {
     uiSettingsOpen = false;
+    // If the admin was live-previewing the default theme, restore their own view.
+    if (adminPreviewActive) restoreOwnTheme();
     writeUrlState();
   }
 }

frontend/js/i18n.js

@@ -65,6 +65,21 @@ const translations = {
     settings_nav_calendars: 'Kalender',
     settings_nav_google: 'Google Konten',
     settings_nav_users: 'Benutzerverwaltung',
+    settings_nav_admin: 'Admin',
+    admin_default_theme: 'Standard-Theme',
+    admin_default_theme_desc: 'Gilt für alle Nutzer ohne eigene Farbe. „Zurück" bei einer Farbe springt auf diesen Standard.',
+    admin_theme_save: 'Standard speichern',
+    admin_theme_discard: 'Verwerfen',
+    admin_theme_saved: 'Standard-Theme gespeichert',
+    admin_branding: 'Branding',
+    admin_branding_desc: 'Eigenes Logo (oben links) und Favicon für die ganze Instanz. PNG/JPEG/WebP, max. 5 MB.',
+    admin_logo: 'Logo',
+    admin_favicon: 'Favicon',
+    admin_upload: 'Hochladen',
+    admin_remove: 'Entfernen',
+    admin_brand_default: 'Standard',
+    admin_branding_saved: 'Branding gespeichert',
+    admin_branding_removed: 'Branding entfernt',
     settings_sync_all: 'Alle synchronisieren',
     settings_sync_all_desc: 'Diese Einstellungen zwischen deinen Geräten teilen',
     settings_sync_this: 'Zwischen Geräten synchronisieren',
@@ -443,6 +458,21 @@ const translations = {
     settings_nav_calendars: 'Calendars',
     settings_nav_google: 'Google Accounts',
     settings_nav_users: 'User Management',
+    settings_nav_admin: 'Admin',
+    admin_default_theme: 'Default theme',
+    admin_default_theme_desc: 'Applies to all users who have not set their own colour. A colour "Reset" returns to this default.',
+    admin_theme_save: 'Save default',
+    admin_theme_discard: 'Discard',
+    admin_theme_saved: 'Default theme saved',
+    admin_branding: 'Branding',
+    admin_branding_desc: 'Custom logo (top-left) and favicon for the whole instance. PNG/JPEG/WebP, max 5 MB.',
+    admin_logo: 'Logo',
+    admin_favicon: 'Favicon',
+    admin_upload: 'Upload',
+    admin_remove: 'Remove',
+    admin_brand_default: 'Default',
+    admin_branding_saved: 'Branding saved',
+    admin_branding_removed: 'Branding removed',
     settings_sync_all: 'Sync all',
     settings_sync_all_desc: 'Share these settings across your devices',
     settings_sync_this: 'Sync across devices',

frontend/js/instance.js

@@ -0,0 +1,56 @@
+// Instance-wide branding + default theme (public GET /api/instance/). Loaded as
+// early as possible so the login screen is already branded. See
+// backend/routers/admin_router.py.
+
+import { setInstanceDefaults } from './settings-sync.js';
+import { setCustomFavicon, applyFavicon } from './utils.js';
+
+export let instanceConfig = {};
+
+// Cache the bundled default logo markup so "remove logo" can restore it.
+const originalLogoHtml = {};
+
+function setBrandLogo(url) {
+  document.querySelectorAll('.topbar-logo, .auth-logo').forEach(el => {
+    const key = el.classList.contains('topbar-logo') ? 'topbar' : 'auth';
+    if (!(key in originalLogoHtml)) originalLogoHtml[key] = el.innerHTML;
+    if (url) {
+      const cls = key === 'topbar' ? 'topbar-logo-img' : 'auth-logo-img';
+      el.innerHTML = `<img class="${cls}" src="${url}" alt="Logo">`;
+    } else {
+      el.innerHTML = originalLogoHtml[key];
+    }
+  });
+}
+
+export function applyInstanceBranding(cfg) {
+  cfg = cfg || {};
+  setInstanceDefaults(cfg.default_theme || {});
+  setCustomFavicon(cfg.has_favicon ? cfg.favicon_url : null);
+  setBrandLogo(cfg.has_logo ? cfg.logo_url : null);
+  applyFavicon();  // no arg → uses instance/built-in primary as fallback
+}
+
+// Memoised so boot() and initCalendar() share a single fetch; callers can await
+// it to guarantee instance defaults are set before the first applyTheme().
+let loadPromise = null;
+export function loadInstance() {
+  if (loadPromise) return loadPromise;
+  loadPromise = (async () => {
+    try {
+      const res = await fetch('/api/instance/', { headers: { Accept: 'application/json' } });
+      instanceConfig = res.ok ? await res.json() : {};
+    } catch (_) {
+      instanceConfig = {};
+    }
+    applyInstanceBranding(instanceConfig);
+    return instanceConfig;
+  })();
+  return loadPromise;
+}
+
+// Force a re-fetch (after an admin changes branding/theme).
+export function reloadInstance() {
+  loadPromise = null;
+  return loadInstance();
+}

frontend/js/settings-sync.js

@@ -92,6 +92,14 @@ export const SETTING_GROUPS = [
 // Flat list of every syncable key the web manages (table order).
 export const SYNCABLE_KEYS = SETTING_GROUPS.flatMap(g => g.rows.map(r => r.key));
 
+// Instance-wide default theme set by an admin (from GET /api/instance/). It is
+// the base a user inherits and the target that a per-colour "Reset" returns to.
+// A user's own value always overrides it. See backend/routers/admin_router.py.
+export let INSTANCE_DEFAULTS = {};
+export function setInstanceDefaults(obj) { INSTANCE_DEFAULTS = obj || {}; }
+// The effective default for a colour key: admin instance default → built-in.
+export function baseColor(key) { return INSTANCE_DEFAULTS[key] || DEFAULT_COLORS[key]; }
+
 const LOCAL_KEY = 'settingsLocal';
 
 export function loadLocal() {

frontend/js/utils.js

@@ -1,4 +1,4 @@
-import { DEFAULT_COLORS } from './settings-sync.js';
+import { DEFAULT_COLORS, INSTANCE_DEFAULTS, baseColor } from './settings-sync.js';
 
 export function isToday(d) {
   const now = new Date();
@@ -106,15 +106,18 @@ export const DEFAULT_BG_COLOR   = DEFAULT_COLORS.bg_color;
 
 export function applyTheme(settings) {
   const root = document.documentElement;
-  root.style.setProperty('--primary',     settings.primary_color || DEFAULT_COLORS.primary_color);
-  root.style.setProperty('--primary-dim', hexToRgba(settings.primary_color || DEFAULT_COLORS.primary_color, 0.15));
-  root.style.setProperty('--accent',      settings.accent_color  || DEFAULT_COLORS.accent_color);
-  root.style.setProperty('--today-color', settings.today_color   || DEFAULT_COLORS.today_color);
+  // Fallback chain for a colour that has no per-user value: admin instance
+  // default → built-in default (baseColor()).
+  const primary = settings.primary_color || baseColor('primary_color');
+  root.style.setProperty('--primary',     primary);
+  root.style.setProperty('--primary-dim', hexToRgba(primary, 0.15));
+  root.style.setProperty('--accent',      settings.accent_color  || baseColor('accent_color'));
+  root.style.setProperty('--today-color', settings.today_color   || baseColor('today_color'));
 
-  // Effektive Farben bestimmen (Override > Default).
-  let textColor = settings.text_color || DEFAULT_TEXT_COLOR;
-  let lineColor = settings.line_color || DEFAULT_LINE_COLOR;
-  let bgColor   = settings.bg_color   || DEFAULT_BG_COLOR;
+  // Effektive Farben bestimmen (Override > Admin-Default > eingebauter Default).
+  let textColor = settings.text_color || baseColor('text_color');
+  let lineColor = settings.line_color || baseColor('line_color');
+  let bgColor   = settings.bg_color   || baseColor('bg_color');
 
   // Sicherheitsbremse: Wenn Schrift- und Hintergrundfarbe nicht genug
   // Kontrast haben (passiert wenn man aus Versehen text=bg eingibt),
@@ -145,43 +148,58 @@ export function applyTheme(settings) {
   const hh = settings.hour_height || 44;
   root.style.setProperty('--hour-h', hh + 'px');
 
-  root.style.setProperty('--month-divider-color', settings.month_divider_color || DEFAULT_COLORS.month_divider_color);
-  root.style.setProperty('--month-label-color',   settings.month_label_color   || DEFAULT_COLORS.month_label_color);
+  root.style.setProperty('--month-divider-color', settings.month_divider_color || baseColor('month_divider_color'));
+  root.style.setProperty('--month-label-color',   settings.month_label_color   || baseColor('month_label_color'));
 
   // Fine-grained element colours. Each is applied ONLY when the user set an
   // explicit value; otherwise the :root default (which references a derived
   // variable) stays in effect, so the look is unchanged until customised.
   // day_selected_color / today_bg_color feed a *-base variable that CSS turns
   // into a subtle tint via color-mix; the rest are applied as-is.
-  const setIf = (varName, value) => { if (value) root.style.setProperty(varName, value); };
-  setIf('--hover-highlight',      settings.hover_highlight_color);
-  setIf('--icon-inactive-color',  settings.icon_inactive_color);
-  setIf('--icon-active-color',    settings.icon_active_color);
-  setIf('--day-hover-color',      settings.day_hover_color);
-  setIf('--day-selected-base',   settings.day_selected_color);
-  setIf('--day-bg',              settings.day_bg_color);
-  setIf('--today-bg-base',       settings.today_bg_color);
+  // User value → admin instance default (if any). When neither is set the
+  // property stays unset so the derived :root default keeps the current look.
+  const setIf = (varName, key) => {
+    const value = settings[key] || INSTANCE_DEFAULTS[key];
+    if (value) root.style.setProperty(varName, value);
+  };
+  setIf('--hover-highlight',     'hover_highlight_color');
+  setIf('--icon-inactive-color', 'icon_inactive_color');
+  setIf('--icon-active-color',   'icon_active_color');
+  setIf('--day-hover-color',     'day_hover_color');
+  setIf('--day-selected-base',   'day_selected_color');
+  setIf('--day-bg',              'day_bg_color');
+  setIf('--today-bg-base',       'today_bg_color');
 }
 
+// Instance custom favicon URL (set by the branding loader). When present it
+// overrides the primary-colour tinting.
+let customFaviconUrl = null;
+export function setCustomFavicon(url) { customFaviconUrl = url || null; }
+
 // Tint the favicon (and browser theme-colour) to the current primary colour so
 // the tab icon reflects the user's theme. Called at load and after saving —
 // NOT on every live keystroke. Reuses the calendar glyph from favicon.svg.
 export function applyFavicon(primaryColor) {
-  const color = primaryColor || DEFAULT_COLORS.primary_color;
-  const svg = `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="${color}" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">`
-    + '<rect x="3" y="4" width="18" height="18" rx="2"/>'
-    + '<line x1="16" y1="2" x2="16" y2="6"/>'
-    + '<line x1="8" y1="2" x2="8" y2="6"/>'
-    + '<line x1="3" y1="10" x2="21" y2="10"/></svg>';
-  const href = 'data:image/svg+xml;base64,' + btoa(svg);
+  const color = primaryColor || baseColor('primary_color');
   let link = document.querySelector('link[rel="icon"]');
   if (!link) {
     link = document.createElement('link');
     link.rel = 'icon';
     document.head.appendChild(link);
   }
+  if (customFaviconUrl) {
+    // Admin-uploaded favicon: use as-is (no primary-colour tinting).
+    link.type = 'image/png';
+    link.href = customFaviconUrl;
+  } else {
+    const svg = `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="${color}" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">`
+      + '<rect x="3" y="4" width="18" height="18" rx="2"/>'
+      + '<line x1="16" y1="2" x2="16" y2="6"/>'
+      + '<line x1="8" y1="2" x2="8" y2="6"/>'
+      + '<line x1="3" y1="10" x2="21" y2="10"/></svg>';
     link.type = 'image/svg+xml';
-  link.href = href;
+    link.href = 'data:image/svg+xml;base64,' + btoa(svg);
+  }
   const meta = document.querySelector('meta[name="theme-color"]');
   if (meta) meta.setAttribute('content', color);
 }

frontend/js/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v84';
+export const APP_VERSION = 'v85';

frontend/sw.js

@@ -7,7 +7,7 @@
 // the entry HTML / version files). New releases take effect on the next
 // reload, no manual SW unregister required.
 
-const CACHE_VERSION  = 'calendarr-v32';
+const CACHE_VERSION  = 'calendarr-v33';
 const OFFLINE_SHELL  = ['/', '/index.html'];
 
 self.addEventListener('install', event => {