[Feature] Allows to set a custom theme for each Wallpaper Engine wallpaper

Describe the enhancement or feature you'd like
Currently, Windows-Auto-Night-Mode can only switch themes based on Windows wallpapers. I would like an option to allow users to assign a custom theme (light or dark) for each Wallpaper Engine wallpaper. Optionally, automatically detect wallpaper brightness to suggest a theme.

Describe alternatives you've considered
Currently, the software cannot detect or respond to Wallpaper Engine wallpapers. Without this feature, users cannot customize theme per Wallpaper Engine wallpaper.

Additional context

Users can manually assign a theme to each Wallpaper Engine wallpaper.

Automatic brightness detection should be optional.

Should not interfere with existing night mode logic.

No files, tests, or entry points are named. Start by locating the existing wallpaper and night-mode logic, then determine how Wallpaper Engine wallpapers can be identified; done means users can assign light or dark themes per wallpaper, optional brightness detection works, and existing night-mode behavior is unchanged.

Written by the indexing model from the issue text.