Enhance the sync modal overlay visibility and aesthetics:
- Add 1px border to content widget using palette(mid) for theme adaptation
- Restore semi-transparent background (alpha=230) to overlay backdrop
- Remove minimum window size constraint (950x550) for better flexibility
The border ensures the overlay is clearly visible in both light and dark
modes, while the semi-transparent background provides visual separation
without completely obscuring the underlying interface
- Added dark.qss stylesheet for dark theme
- Implemented toggleTheme() slot in PalladiumGUI
- Added 'Dark Mode' checkbox to Settings menu
- Theme preference is saved in QSettings (darkModeEnabled)
- Updated palladium.qrc to include style resources
