diff --git a/client/src/App.tsx b/client/src/App.tsx
index a4e4761..c58d526 100644
--- a/client/src/App.tsx
+++ b/client/src/App.tsx
@@ -29,6 +29,7 @@ import {
   resolveColorScheme,
   subscribeToSystemColorScheme
 } from './services/appearance.js'
+import { syncAppearancePrefs } from './services/appearancePrefs.js'
 import { startBackgroundSync, stopBackgroundSync, syncAllLogbooks, subscribeToSyncState } from './services/sync.js'
 import ReadOnlyViewer from './components/ReadOnlyViewer.tsx'
 import DemoViewer from './components/DemoViewer.tsx'
@@ -151,6 +152,13 @@ function App() {
     })
   }, [])
 
+  useEffect(() => {
+    if (!isAuthenticated) return
+    const userId = localStorage.getItem('active_userid')
+    if (!userId) return
+    void syncAppearancePrefs(userId)
+  }, [isAuthenticated])
+
   useEffect(() => {
     const handleOnline = () => {
       setOnline(true)
diff --git a/client/src/components/UserProfilePreferences.tsx b/client/src/components/UserProfilePreferences.tsx
index f0b8f1a..252a716 100644
--- a/client/src/components/UserProfilePreferences.tsx
+++ b/client/src/components/UserProfilePreferences.tsx
@@ -5,6 +5,7 @@ import ThemedSelect from './ThemedSelect.tsx'
 import PushNotificationSettings from './PushNotificationSettings.tsx'
 import PwaInstallPrompt from './PwaInstallPrompt.tsx'
 import { notifyAppearanceChanged } from '../services/appearance.js'
+import { saveAppearancePrefsToServer } from '../services/appearancePrefs.js'
 import { useAppTour } from '../context/AppTourContext.tsx'
 import {
   getColorSchemePreference,
@@ -32,6 +33,9 @@ export default function UserProfilePreferences({ userId }: UserProfilePreference
     setThemePreference(userId, nextTheme)
     setColorSchemePreference(userId, nextColorScheme)
     notifyAppearanceChanged()
+    void saveAppearancePrefsToServer(nextTheme, nextColorScheme).catch((err) => {
+      console.warn('Failed to save appearance prefs to server:', err)
+    })
   }
 
   const handleThemeChange = (nextTheme: string) => {
diff --git a/client/src/services/appearancePrefs.test.ts b/client/src/services/appearancePrefs.test.ts
new file mode 100644
index 0000000..87c8912
--- /dev/null
+++ b/client/src/services/appearancePrefs.test.ts
@@ -0,0 +1,72 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import {
+  fetchAppearancePrefs,
+  saveAppearancePrefsToServer,
+  syncAppearancePrefs
+} from './appearancePrefs.js'
+import { setThemePreference } from './userPreferences.js'
+
+const USER_ID = 'appearance-sync-user'
+
+vi.mock('./api.js', () => ({
+  apiJson: vi.fn()
+}))
+
+import { apiJson } from './api.js'
+
+const mockedApiJson = vi.mocked(apiJson)
+
+describe('appearancePrefs', () => {
+  beforeEach(() => {
+    localStorage.clear()
+    vi.clearAllMocks()
+  })
+
+  it('fetchAppearancePrefs returns defaults when not authenticated', async () => {
+    await expect(fetchAppearancePrefs()).resolves.toEqual({
+      theme: 'auto',
+      colorScheme: 'auto',
+      persisted: false
+    })
+    expect(mockedApiJson).not.toHaveBeenCalled()
+  })
+
+  it('syncAppearancePrefs applies server prefs after cache wipe', async () => {
+    localStorage.setItem('active_userid', USER_ID)
+    mockedApiJson.mockResolvedValueOnce({
+      theme: 'ocean',
+      colorScheme: 'dark',
+      persisted: true
+    })
+
+    const changed = vi.fn()
+    window.addEventListener('appearance-changed', changed)
+
+    await syncAppearancePrefs(USER_ID)
+
+    expect(localStorage.getItem(`user_pref_theme_${USER_ID}`)).toBe('ocean')
+    expect(localStorage.getItem(`user_pref_color_scheme_${USER_ID}`)).toBe('dark')
+    expect(changed).toHaveBeenCalledTimes(1)
+  })
+
+  it('syncAppearancePrefs uploads local prefs when server has none', async () => {
+    localStorage.setItem('active_userid', USER_ID)
+    setThemePreference(USER_ID, 'material')
+    mockedApiJson
+      .mockResolvedValueOnce({ theme: 'auto', colorScheme: 'auto', persisted: false })
+      .mockResolvedValueOnce({ theme: 'material', colorScheme: 'auto', persisted: true })
+
+    await syncAppearancePrefs(USER_ID)
+
+    expect(mockedApiJson).toHaveBeenCalledTimes(2)
+    expect(mockedApiJson).toHaveBeenLastCalledWith('/api/auth/appearance-prefs', {
+      method: 'PUT',
+      body: JSON.stringify({ theme: 'material', colorScheme: 'auto' })
+    })
+  })
+
+  it('saveAppearancePrefsToServer skips when not authenticated', async () => {
+    await saveAppearancePrefsToServer('ocean', 'light')
+    expect(mockedApiJson).not.toHaveBeenCalled()
+  })
+})
diff --git a/client/src/services/appearancePrefs.ts b/client/src/services/appearancePrefs.ts
new file mode 100644
index 0000000..4a24261
--- /dev/null
+++ b/client/src/services/appearancePrefs.ts
@@ -0,0 +1,62 @@
+import { apiJson } from './api.js'
+import { notifyAppearanceChanged } from './appearance.js'
+import {
+  getActiveUserId,
+  getColorSchemePreference,
+  getThemePreference,
+  setColorSchemePreference,
+  setThemePreference
+} from './userPreferences.js'
+
+const API_BASE = '/api/auth/appearance-prefs'
+
+export interface AppearancePrefs {
+  theme: string
+  colorScheme: string
+  persisted: boolean
+}
+
+function hasLocalAppearancePrefs(userId: string): boolean {
+  return (
+    localStorage.getItem(`user_pref_theme_${userId}`) != null ||
+    localStorage.getItem(`user_pref_color_scheme_${userId}`) != null
+  )
+}
+
+export async function fetchAppearancePrefs(): Promise<AppearancePrefs> {
+  if (!getActiveUserId()) {
+    return { theme: 'auto', colorScheme: 'auto', persisted: false }
+  }
+
+  return apiJson<AppearancePrefs>(API_BASE)
+}
+
+export async function saveAppearancePrefsToServer(theme: string, colorScheme: string): Promise<void> {
+  if (!getActiveUserId()) return
+
+  await apiJson<AppearancePrefs>(API_BASE, {
+    method: 'PUT',
+    body: JSON.stringify({ theme, colorScheme })
+  })
+}
+
+/** Merge server-stored appearance with local cache (server wins after cache wipe). */
+export async function syncAppearancePrefs(userId?: string | null): Promise<void> {
+  const id = userId?.trim() || getActiveUserId()
+  if (!id) return
+
+  try {
+    const server = await fetchAppearancePrefs()
+
+    if (server.persisted) {
+      setThemePreference(id, server.theme)
+      setColorSchemePreference(id, server.colorScheme)
+    } else if (hasLocalAppearancePrefs(id)) {
+      await saveAppearancePrefsToServer(getThemePreference(id), getColorSchemePreference(id))
+    }
+  } catch (err) {
+    console.warn('Failed to sync appearance preferences:', err)
+  }
+
+  notifyAppearanceChanged()
+}
diff --git a/server/prisma/schema.prisma b/server/prisma/schema.prisma
index e0dffcc..ab8b89f 100644
--- a/server/prisma/schema.prisma
+++ b/server/prisma/schema.prisma
@@ -22,6 +22,7 @@ model User {
   collaborations           Collaboration[]
   pushSubscriptions        PushSubscription[]
   notificationPrefs        UserNotificationPrefs?
+  appearancePrefs          UserAppearancePrefs?
 }
 
 model PushSubscription {
@@ -48,6 +49,15 @@ model UserNotificationPrefs {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 }
 
+model UserAppearancePrefs {
+  userId      String   @id
+  theme       String   @default("auto")
+  colorScheme String   @default("auto")
+  updatedAt   DateTime @updatedAt
+
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+}
+
 model Credential {
   id           String   @id @default(uuid())
   userId       String
diff --git a/server/src/routes/auth.ts b/server/src/routes/auth.ts
index 5c1c07b..237fa38 100644
--- a/server/src/routes/auth.ts
+++ b/server/src/routes/auth.ts
@@ -38,6 +38,17 @@ function normalizeCredentialLabel(label: unknown): string | null {
   return trimmed.slice(0, 64)
 }
 
+const VALID_THEMES = new Set(['auto', 'ocean', 'material', 'cupertino'])
+const VALID_COLOR_SCHEMES = new Set(['auto', 'light', 'dark'])
+
+function parseThemePreference(value: unknown): string | null {
+  return typeof value === 'string' && VALID_THEMES.has(value) ? value : null
+}
+
+function parseColorSchemePreference(value: unknown): string | null {
+  return typeof value === 'string' && VALID_COLOR_SCHEMES.has(value) ? value : null
+}
+
 router.post('/register-options', async (req, res) => {
   try {
     const { username } = req.body
@@ -426,6 +437,57 @@ router.post('/rotate-recovery', requireReauth, async (req: any, res) => {
   }
 })
 
+router.get('/appearance-prefs', requireUser, async (req: any, res) => {
+  try {
+    const prefs = await prisma.userAppearancePrefs.findUnique({
+      where: { userId: req.userId }
+    })
+
+    return res.json({
+      theme: prefs?.theme ?? 'auto',
+      colorScheme: prefs?.colorScheme ?? 'auto',
+      persisted: prefs != null
+    })
+  } catch (error: any) {
+    console.error('Error reading appearance prefs:', error)
+    return res.status(500).json({ error: error.message || 'Internal server error' })
+  }
+})
+
+router.put('/appearance-prefs', requireUser, async (req: any, res) => {
+  try {
+    const theme = parseThemePreference(req.body?.theme)
+    const colorScheme = parseColorSchemePreference(req.body?.colorScheme)
+    if (!theme || !colorScheme) {
+      return res.status(400).json({ error: 'Invalid theme or colorScheme' })
+    }
+
+    const prefs = await prisma.userAppearancePrefs.upsert({
+      where: { userId: req.userId },
+      create: {
+        userId: req.userId,
+        theme,
+        colorScheme,
+        updatedAt: new Date()
+      },
+      update: {
+        theme,
+        colorScheme,
+        updatedAt: new Date()
+      }
+    })
+
+    return res.json({
+      theme: prefs.theme,
+      colorScheme: prefs.colorScheme,
+      persisted: true
+    })
+  } catch (error: any) {
+    console.error('Error updating appearance prefs:', error)
+    return res.status(500).json({ error: error.message || 'Internal server error' })
+  }
+})
+
 router.get('/profile', requireUser, async (req: any, res) => {
   try {
     const user = await prisma.user.findUnique({