- Add new theme IDs (nord, dracula, gruvbox, tokyo-night) to core types and ThemeSelector component
- Add CSS custom properties for all four themes covering background, text, accent, border, and surface colors
- Add theme validation sync, bootstrap logic, and unit tests for ThemeSelector and useTheme hook
- Update GitManagerModal to support new theme-aware styling
- Update README theme system documentation with expanded theme list
- Add changeset for published package