diff --git a/backend/migrations/core/137_add_slideshow_share.js b/backend/migrations/core/138_add_slideshow_share.js
similarity index 100%
rename from backend/migrations/core/137_add_slideshow_share.js
rename to backend/migrations/core/138_add_slideshow_share.js
diff --git a/backend/migrations/core/138_add_slideshow_styling.js b/backend/migrations/core/139_add_slideshow_styling.js
similarity index 97%
rename from backend/migrations/core/138_add_slideshow_styling.js
rename to backend/migrations/core/139_add_slideshow_styling.js
index 4a159d1e..df86c98a 100644
--- a/backend/migrations/core/138_add_slideshow_styling.js
+++ b/backend/migrations/core/139_add_slideshow_styling.js
@@ -1,6 +1,6 @@
 const { addColumnIfNotExists } = require('../helpers');
 
-// Live Slideshow styling (migration 137 follow-up):
+// Live Slideshow styling (migration 138 follow-up):
 //   - a ZDF/ARD-ident-style watermark: a white, semi-transparent logo in a
 //     corner of the projected slideshow (sourced from the site branding logo
 //     or the event's own logo).
@@ -54,5 +54,5 @@ exports.up = async function up(knex) {
 };
 
 exports.down = async function down() {
-  // Safe rollback - intentionally no-op to avoid data loss (mirrors 074/137).
+  // Safe rollback - intentionally no-op to avoid data loss (mirrors 074/138).
 };
diff --git a/backend/src/routes/adminEvents.js b/backend/src/routes/adminEvents.js
index 507348e7..a1cc3f10 100644
--- a/backend/src/routes/adminEvents.js
+++ b/backend/src/routes/adminEvents.js
@@ -1882,7 +1882,7 @@ router.post('/:id/toggle-status', adminAuth, requirePermission('events.edit'), r
 
 // ---------------------------------------------------------------------------
 // Live Slideshow ("Diashow") — a token-only fullscreen kiosk link for live
-// events that auto-picks-up new uploads (migration 137). Mirrors the
+// events that auto-picks-up new uploads (migration 138). Mirrors the
 // client-access second-token pattern: the link is minted on demand, rotatable
 // and disable-able, independent of the gallery password / share link.
 // ---------------------------------------------------------------------------
diff --git a/backend/src/routes/adminSettings.js b/backend/src/routes/adminSettings.js
index 1eadc09d..3bb261a7 100644
--- a/backend/src/routes/adminSettings.js
+++ b/backend/src/routes/adminSettings.js
@@ -290,7 +290,7 @@ router.put('/accounting', adminAuth, requirePermission('settings.edit'), async (
   }
 });
 
-// Global Live Slideshow defaults (migration 138). The per-event watermark is
+// Global Live Slideshow defaults (migration 139). The per-event watermark is
 // tri-state (events.show_watermark NULL = inherit these). Read via the generic
 // GET /:type ('slideshow'); this is the typed write.
 router.put('/slideshow', adminAuth, requirePermission('settings.edit'), async (req, res) => {
diff --git a/backend/src/routes/gallery.js b/backend/src/routes/gallery.js
index 888e8d70..1187185d 100644
--- a/backend/src/routes/gallery.js
+++ b/backend/src/routes/gallery.js
@@ -220,7 +220,7 @@ router.get('/:slug/info', async (req, res) => {
 
 // ---------------------------------------------------------------------------
 // Live Slideshow ("Diashow") — token-only fullscreen kiosk surface
-// (migration 137). The token in the URL IS the secret (no gallery password),
+// (migration 138). The token in the URL IS the secret (no gallery password),
 // so these routes are unauthenticated except for the token match itself. The
 // slideshow shows ALL public/visible, finished photos — exactly the guest
 // set — so once /session mints a short-lived `accessLevel:'slideshow'` JWT,
@@ -586,7 +586,7 @@ router.get('/:slug/photos', verifyGalleryAccess, resolveGuest, async (req, res)
     // Log view — but NOT for the Live Slideshow kiosk. A running projector
     // refetches this list on every new-upload poll, which would massively
     // inflate total_views / unique_visitors. The slideshow is explicitly
-    // excluded from real visitor analytics (migration 137 design).
+    // excluded from real visitor analytics (migration 138 design).
     if (req.accessLevel !== 'slideshow') {
       await db('access_logs').insert({
         event_id: req.event.id,
diff --git a/frontend/src/components/admin/SlideshowSettingsCard.tsx b/frontend/src/components/admin/SlideshowSettingsCard.tsx
index b568dd14..ada57c3b 100644
--- a/frontend/src/components/admin/SlideshowSettingsCard.tsx
+++ b/frontend/src/components/admin/SlideshowSettingsCard.tsx
@@ -1,7 +1,7 @@
 /**
  * <SlideshowSettingsCard>
  *
- * Per-event "Live Slideshow" ("Diashow") admin surface (migrations 137/138).
+ * Per-event "Live Slideshow" ("Diashow") admin surface (migrations 138/139).
  * A token-only fullscreen kiosk link for live events that auto-picks-up new
  * uploads while it runs. Mounted once on the EventDetailsPage; admin can:
  *   - Generate the slideshow link on demand (mints show_share_token)
diff --git a/frontend/src/pages/admin/EventDetailsPage.tsx b/frontend/src/pages/admin/EventDetailsPage.tsx
index 9b5acba0..079a8b73 100644
--- a/frontend/src/pages/admin/EventDetailsPage.tsx
+++ b/frontend/src/pages/admin/EventDetailsPage.tsx
@@ -2159,7 +2159,7 @@ export const EventDetailsPage: React.FC = () => {
             </div>
           </Card>
 
-          {/* Live Slideshow ("Diashow") link + live display settings (migrations 137/138).
+          {/* Live Slideshow ("Diashow") link + live display settings (migrations 138/139).
               Gated behind the `slideshow` feature flag. */}
           {flags.slideshow && (
           <SlideshowSettingsCard
diff --git a/frontend/src/pages/admin/__tests__/brandingCustomCss.test.tsx b/frontend/src/pages/admin/__tests__/brandingCustomCss.test.tsx
new file mode 100644
index 00000000..bd4af330
--- /dev/null
+++ b/frontend/src/pages/admin/__tests__/brandingCustomCss.test.tsx
@@ -0,0 +1,90 @@
+import { render, screen, waitFor } from '@testing-library/react';
+import { vi } from 'vitest';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+
+import { GALLERY_THEME_PRESETS } from '../../../types/theme.types';
+
+const CSS = '.gallery-premium-footer p:first-child { display: none; }';
+const savedTheme = { ...GALLERY_THEME_PRESETS.galleryPremium.config, customCss: CSS };
+
+// --- mocks -----------------------------------------------------------------
+vi.mock('react-i18next', async () => {
+  const actual = await vi.importActual<typeof import('react-i18next')>('react-i18next');
+  return {
+    ...actual,
+    useTranslation: () => ({ t: (k: string, fb?: unknown) => (typeof fb === 'string' ? fb : k) })
+  };
+});
+
+vi.mock('react-toastify', () => ({ toast: { success: vi.fn(), error: vi.fn() } }));
+
+vi.mock('../../../services/settings.service', async () => {
+  const actual = await vi.importActual<any>('../../../services/settings.service');
+  return {
+    ...actual,
+    settingsService: {
+      ...actual.settingsService,
+      getSettingsByType: vi.fn(async (type: string) =>
+        type === 'theme' ? { theme_config: savedTheme } : {}
+      ),
+      getAllSettings: vi.fn(async () => ({ thumbnail_width: '800', thumbnail_height: '800' })),
+      updateTheme: vi.fn(async () => undefined),
+      updateBranding: vi.fn(async () => undefined),
+    },
+  };
+});
+
+vi.mock('../../../services/fonts.service', () => ({
+  fontsService: { list: vi.fn(async () => []) },
+  extractFamilyName: (s: string) => s,
+}));
+
+vi.mock('../../../services/businessProfile.service', () => ({
+  businessProfileService: { get: vi.fn(async () => ({ profile: {} })), update: vi.fn() },
+}));
+
+vi.mock('../../../hooks/usePublicSettings', () => ({
+  PUBLIC_SETTINGS_QUERY_KEY: ['public-settings'],
+  usePublicSettings: () => ({ data: { branding_force_color_mode: null } }),
+}));
+
+vi.mock('../../../contexts/FeatureFlagsContext', () => ({
+  useFeatureFlags: () => ({ flags: {} }),
+  useFeatureEnabled: () => false,
+}));
+
+// Heavy/unrelated children — stub to keep the test focused on the CSS textarea.
+vi.mock('../../../components/admin/CustomerDashboardBrandingCard', () => ({
+  CustomerDashboardBrandingCard: () => null,
+}));
+vi.mock('../../../components/admin/PdfTypographyCard', () => ({
+  PdfTypographyCard: () => null,
+}));
+vi.mock('../../../components/admin', async () => {
+  const actual = await vi.importActual<any>('../../../components/admin');
+  return { ...actual, GalleryPreview: () => null };
+});
+
+import { BrandingPage } from '../BrandingPage';
+import { ThemeProvider } from '../../../contexts/ThemeContext';
+
+describe('BrandingPage custom CSS persistence (#645)', () => {
+  beforeEach(() => localStorage.clear());
+
+  it('shows the saved custom CSS in the textarea on load for a Gallery Premium theme', async () => {
+    const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+    render(
+      <QueryClientProvider client={qc}>
+        <ThemeProvider>
+          <BrandingPage />
+        </ThemeProvider>
+      </QueryClientProvider>
+    );
+
+    const textarea = (await screen.findByPlaceholderText(
+      '/* Add custom CSS here */'
+    )) as HTMLTextAreaElement;
+
+    await waitFor(() => expect(textarea.value).toBe(CSS));
+  });
+});
diff --git a/frontend/src/types/index.ts b/frontend/src/types/index.ts
index ce5895f7..b84491bf 100644
--- a/frontend/src/types/index.ts
+++ b/frontend/src/types/index.ts
@@ -67,7 +67,7 @@ export interface Event {
   // Client access (#172)
   client_access_enabled?: boolean;
   client_share_token?: string;
-  // Live Slideshow / "Diashow" (migration 137). Token-only fullscreen kiosk
+  // Live Slideshow / "Diashow" (migration 138). Token-only fullscreen kiosk
   // link minted on demand; null token = disabled. Settings drive the running
   // projector and can be changed live.
   show_share_token?: string | null;