[Desktop > Appearance] Tweak app window corner radius switching

[git-f0x]

Low priority:
Currently, the app window corners use the same radius on Square and Slightly Round, and a different (larger) radius on Round.
The NavBar (and some other components), use the same radius on Round and Slightly Round, and a different radius on Square.
The app window corners should match the NavBar switching behavior, and use the same radius as Round on Slightly Round, while keeping the current radius on Square.
_{This was approved by Maria.}