Add a ThemeContext and provider, wrap the app with ThemeProvider, and sync user settings' theme into the context. Replace hardcoded color classes with design token classes (background, muted, foreground, border, card, etc.) across multiple UI components to centralize theming and enable consistent light/dark styling. Files updated include App.tsx (useTheme, setTheme, ThemeProvider, footer/background tokens), several views and components (AddMediaView, BrowseView, CastDetailView, CastView, MediaCard, MediaListItem, SettingsView, ImporterView) to use tokenized classes, and add new src/contexts/ThemeContext.tsx.
