docs: update README with gitea-dark as default theme

Note that custom themes are available for manual selection but may not work as default

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

dev/README.md

@@ -27,7 +27,9 @@ To enable the Gitea Actions runner:
 
 ## Custom Themes
 
-21 custom themes are available from [sainnhe/gitea-themes](https://git.sainnhe.dev/sainnhe/gitea-themes):
+**Default Theme:** `gitea-dark` (built-in Gitea dark theme)
+
+21 custom themes from [sainnhe/gitea-themes](https://git.sainnhe.dev/sainnhe/gitea-themes) are available for manual selection:
 
 - Edge (auto, dark, light)
 - Everforest (auto, dark, light)
@@ -38,6 +40,8 @@ To enable the Gitea Actions runner:
 
 Users can select their preferred theme in **Settings** → **Appearance**.
 
+**Note:** Custom themes may not load as default theme due to Gitea's theme loading mechanism. They remain available for manual selection by users.
+
 ## Custom Logo & Favicon
 
 Custom logo and favicon are mounted from `dev/custom/public/assets/img/`: