Add Dark Theme support

[mtpontes]

Implementation Details

• The AppTheme class loads color values based on the AppConfig.DarkTheme setting on application startup.
• Changing the theme requires an application restart to fully re-initialize all UI components with the new color scheme. A prompt is displayed to the user upon toggling the setting.
• Used a consolidation approach for AppTheme within the Methods namespace to ensure compatibility with the existing project structure and build system.

How to Test

1. Launch the application.
2. Go to About (Top-right toolbar icon) > App Settings (Sidebar gear icon).
3. Scroll to find the "Dark Theme (Dark Mode)" checkbox.
4. Check the box and accept the restart prompt.
5. Verify that the application restarts with the Dark Theme applied to all main areas (Sidebar, Content Area, Toolbar).

Screenshots