- Add global CSS rules for select elements in dark mode
- Use color-scheme property to signal dark mode to browser
- Explicitly style option elements with dark theme colors
- Fix UsageStats dropdown to use correct CSS variables (bg-surface, text-text-main)
Fixes dropdown text being unreadable in dark theme on usage page:
- Provider filter dropdown
- Table view selector (Model/Account/API Key/Endpoint)
- Pagination page size selector
Tested in Chrome and Firefox with both light and dark themes.