- Add CSS custom property token system to globals.css (dark + light variants)
- Add anti-FOUC inline script in layout.tsx head to apply stored theme before hydration
- Create useTheme hook (src/hooks/useTheme.ts) backed by usePersistentState
- Add Sun/Moon toggle button to LeftToolbar above Settings
- Wire useTheme into ThreePanelLayout and pass theme/onThemeToggle to LeftToolbar
- Migrate hardcoded hex values to CSS vars in LeftToolbar and ThreePanelLayout
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>