## 1. Theme Foundation

- [x] 1.1 Define root-level theme state model for system, light, dark, and high-contrast
- [x] 1.2 Add CSS token/variable mapping so all theme modes can be resolved consistently

## 2. Theme Switcher UI

- [x] 2.1 Add icon-based theme switcher control to top-right header area
- [x] 2.2 Provide accessible labels and active-state indication for each icon option

## 3. Theme Preference Persistence

- [x] 3.1 Persist selected theme in localStorage when available
- [x] 3.2 Implement cookie fallback persistence when localStorage is unavailable
- [x] 3.3 Restore persisted preference for returning users

## 4. System Default Behavior

- [x] 4.1 Apply system mode when no persisted preference exists
- [x] 4.2 Ensure saved user preference overrides system default on subsequent visits

## 5. Validation and Documentation

- [x] 5.1 Validate theme switching and persistence across refreshes and browser restarts
- [x] 5.2 Validate icon controls with keyboard navigation and screen reader labels
- [x] 5.3 Update README/docs with theme options and persistence behavior
