Allow us to set the Background Color for light and dark wallpapers

**Describe the enhancement or feature you'd like**
When setting a wallpaper, Windows allows users to set a background color too. This background color is used to fill any areas not filled by the wallpaper.

AutoDark can automatically adjust the wallpaper based on Light and Dark mode, but it lacks an option to change the Background color for each. This means the desktop can get mismatched colors when the user has a custom background color set.

**Additional context**
See images below for an example.

**A mismatched desktop because the background color is set for light, but can not be updated by Auto Dark when switching to a dark background:**

**The setting in windows:**

No file or test is named in the issue. Start by locating the wallpaper-setting entry point and how Windows exposes the per-mode background color. Done means AutoDark can store and apply separate light and dark background colors when switching wallpapers, including unfilled wallpaper areas.

Written by the indexing model from the issue text.