fix(branding): preserve customCss through preset switches + theme changes (#645)

Reporter @aemisrogers nailed the root cause: same #317 class of bug as
logoUrl. None of `GALLERY_THEME_PRESETS` (`theme.types.ts:125`) include
`customCss` in their `config` object, so any path that REPLACES
`currentTheme` with `preset.config` (or with a sparse `newTheme` that
came from `preset.config` upstream) silently dropped `customCss` from
React state. The persisted value in `theme_config` stayed correct (the
public gallery still rendered it), but the admin textarea showed
empty on reload — admin-UI display drift, not data loss.

Three surgical fixes, mirroring the #317 logoUrl pattern:

1. `BrandingPage.tsx` `handleThemeChange` — `customCss: newTheme.customCss
   ?? currentTheme.customCss` alongside the existing `logoUrl` fallback.
   Closes the propagation hole where the customizer's `handlePresetSelect`
   fires `onChange(preset.config)` (no customCss) and the parent wipes
   it from currentTheme.

2. `BrandingPage.tsx` `handlePresetChange` — preserve `customCss` from
   prev/currentTheme on preset switch, same shape as the existing
   `logoUrl: prev.logoUrl` preservation. Touches both the `setCurrentTheme`
   and the preview-mode `setTheme` paths.

3. `ThemeCustomizerEnhanced.tsx` `handlePresetSelect` — remove the
   `setCustomCss('')` that wiped the local textarea state on preset
   pick. The previous comment ("Clear custom CSS when selecting a preset")
   described the original intent but produced data drift across the
   preset round-trip. The sibling `ThemeCustomizer.tsx` already never
   cleared it; this aligns the two.

Verified against `v3.44.0` and `origin/beta`: identical code on both
branches, so the bug exists on stable + beta. Lint + tsc clean on the
two changed files.

Closes #645.

frontend/src/components/admin/ThemeCustomizerEnhanced.tsx

@@ -259,7 +259,13 @@ export const ThemeCustomizerEnhanced: React.FC<ThemeCustomizerEnhancedProps> = (
     if (preset) {
       setSelectedPreset(presetKey);
       setLocalTheme(preset.config);
-      setCustomCss(''); // Clear custom CSS when selecting a preset
+      // Don't wipe customCss on preset pick — preset configs carry no
+      // customCss, and the admin's persisted styling extras should
+      // survive a layout switch (#645). Matches ThemeCustomizer.tsx
+      // which never cleared it. The parent's handleThemeChange merges
+      // via `customCss: newTheme.customCss ?? currentTheme.customCss`,
+      // so propagating preset.config (no customCss) keeps the saved
+      // value intact end-to-end.
       if (onPresetChange) {
         onPresetChange(presetKey);
       }

frontend/src/pages/admin/BrandingPage.tsx

@@ -186,12 +186,14 @@ export const BrandingPage: React.FC = () => {
   };
 
   const handleThemeChange = (newTheme: ThemeConfig) => {
-    // Preset configs don't carry a logoUrl, so a preset change inside the
+    // Preset configs don't carry a logoUrl or customCss, so a preset change
-    // customizer arrives here with newTheme.logoUrl=undefined. Keep the
+    // inside the customizer arrives here with those fields undefined. Keep
-    // existing logo instead of wiping branding_logo_url on save (#317).
+    // the existing values instead of wiping the persisted ones on save
+    // (#317 for logoUrl, #645 for customCss).
     const mergedTheme: ThemeConfig = {
       ...newTheme,
-      logoUrl: newTheme.logoUrl ?? currentTheme.logoUrl
+      logoUrl: newTheme.logoUrl ?? currentTheme.logoUrl,
+      customCss: newTheme.customCss ?? currentTheme.customCss
     };
     setCurrentTheme(mergedTheme);
     if (newTheme.logoUrl !== undefined && newTheme.logoUrl !== currentTheme.logoUrl) {
@@ -207,10 +209,20 @@ export const BrandingPage: React.FC = () => {
     // Get the preset theme config
     const preset = GALLERY_THEME_PRESETS[presetName];
     if (preset) {
-      // Preserve the existing logo when switching presets (#317).
+      // Preserve the existing logo + custom CSS when switching presets
-      setCurrentTheme(prev => ({ ...preset.config, logoUrl: prev.logoUrl }));
+      // (#317 for logo, #645 for customCss). Presets define a look; they
+      // shouldn't silently drop the admin's persisted styling extras.
+      setCurrentTheme(prev => ({
+        ...preset.config,
+        logoUrl: prev.logoUrl,
+        customCss: prev.customCss
+      }));
       if (isPreviewMode) {
-        setTheme({ ...preset.config, logoUrl: currentTheme.logoUrl });
+        setTheme({
+          ...preset.config,
+          logoUrl: currentTheme.logoUrl,
+          customCss: currentTheme.customCss
+        });
       }
     }
   };