[Feature] Allow Custom CSS / Theme Styling & Plugin System for UI Customization

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request. • Yes The feature Please consider adding support for custom user styling and/or an extensible plugin system to give users the freedom to customize the UI/UX. Proposed Solutions: • Custom CSS/Theme Settings: Allow users and server administrators to inject custom CSS or select/create custom themes (color accents, fonts, border radii, card styles) directly from the Web UI settings. • Plugin / Extension Architecture: Introduce a modular plugin or theming hooks mechanism where community-created UI themes, layouts, or components can be loaded without modifying core code. • Custom CSS Variables (Design Tokens): Expose primary theme tokens as accessible CSS variables so tweaking styling across the interface is simple and persistent across updates. Use Case & Context: Self-hosted users often prefer tailoring their photo gallery aesthetic to…