diff --git a/client/src/services/appearancePrefs.test.ts b/client/src/services/appearancePrefs.test.ts
index 87c8912..ba95856 100644
--- a/client/src/services/appearancePrefs.test.ts
+++ b/client/src/services/appearancePrefs.test.ts
@@ -69,4 +69,28 @@ describe('appearancePrefs', () => {
     await saveAppearancePrefsToServer('ocean', 'light')
     expect(mockedApiJson).not.toHaveBeenCalled()
   })
+
+  it('syncAppearancePrefs skips server sync when userId does not match active session', async () => {
+    localStorage.setItem('active_userid', 'session-user')
+    setThemePreference('other-user', 'ocean')
+    mockedApiJson.mockResolvedValue({
+      theme: 'material',
+      colorScheme: 'dark',
+      persisted: true
+    })
+
+    await syncAppearancePrefs('other-user')
+
+    expect(mockedApiJson).not.toHaveBeenCalled()
+    expect(localStorage.getItem('user_pref_theme_other-user')).toBe('ocean')
+  })
+
+  it('syncAppearancePrefs skips server sync when active session is missing', async () => {
+    setThemePreference(USER_ID, 'ocean')
+
+    await syncAppearancePrefs(USER_ID)
+
+    expect(mockedApiJson).not.toHaveBeenCalled()
+    expect(localStorage.getItem(`user_pref_theme_${USER_ID}`)).toBe('ocean')
+  })
 })
diff --git a/client/src/services/appearancePrefs.ts b/client/src/services/appearancePrefs.ts
index 4a24261..e55f87f 100644
--- a/client/src/services/appearancePrefs.ts
+++ b/client/src/services/appearancePrefs.ts
@@ -23,16 +23,30 @@ function hasLocalAppearancePrefs(userId: string): boolean {
   )
 }
 
-export async function fetchAppearancePrefs(): Promise<AppearancePrefs> {
-  if (!getActiveUserId()) {
+function resolveSyncedUserId(userId?: string | null): string | null {
+  const id = userId?.trim() || getActiveUserId()?.trim() || null
+  if (!id) return null
+
+  const activeId = getActiveUserId()?.trim() || null
+  if (!activeId || activeId !== id) return null
+
+  return id
+}
+
+export async function fetchAppearancePrefs(userId?: string | null): Promise<AppearancePrefs> {
+  if (!resolveSyncedUserId(userId)) {
     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
+export async function saveAppearancePrefsToServer(
+  theme: string,
+  colorScheme: string,
+  userId?: string | null
+): Promise<void> {
+  if (!resolveSyncedUserId(userId)) return
 
   await apiJson<AppearancePrefs>(API_BASE, {
     method: 'PUT',
@@ -42,17 +56,17 @@ export async function saveAppearancePrefsToServer(theme: string, colorScheme: st
 
 /** 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()
+  const id = resolveSyncedUserId(userId)
   if (!id) return
 
   try {
-    const server = await fetchAppearancePrefs()
+    const server = await fetchAppearancePrefs(id)
 
     if (server.persisted) {
       setThemePreference(id, server.theme)
       setColorSchemePreference(id, server.colorScheme)
     } else if (hasLocalAppearancePrefs(id)) {
-      await saveAppearancePrefsToServer(getThemePreference(id), getColorSchemePreference(id))
+      await saveAppearancePrefsToServer(getThemePreference(id), getColorSchemePreference(id), id)
     }
   } catch (err) {
     console.warn('Failed to sync appearance preferences:', err)