Feature request: Switch different filling modes between different modes

**Describe the enhancement or feature you'd like**
Switching between bright and dark mode should also switch the fill mode.

Accordingly it should move down to the wallpaper section

**Describe alternatives you've considered**
Converting all of my wallpapers to 16:10.

**Additional context**
Neither of my wallpapers are 16:10, which means I always either have ugly borders where I don't want them, or a blurry mess.

What I want:
This is a 1:1 image, so I want
Fill mode: center, "Anpassen"

And this is a 16:9 screenshot, so I want the sides cut off with
Fill mode: fill, "Füllen"

No file or test is named in the issue. Start by tracing the wallpaper settings and the bright/dark mode switch, then determine how fill mode is stored and applied; done means each mode can use the requested fill behavior for the corresponding wallpaper without unwanted borders or blur.

Written by the indexing model from the issue text.