Disable custom backgrounds with dark mode enabled

With dark mode enabled all kinds of wonky drawing effects show up if you also set custom backgrounds.

Easiest fix is to disable this combination in the settings and to disallow it in the backend.

changed milestone to %Xfce 4.16

Seems to have been a red herring, it actually works fine (plus it is helpful because dark mode changes the foreground color, so when setting a dark gradient all contrasts still look ok).

closed

removed milestone %Xfce 4.16

added 5. Background Color / Image 5. Visual Appearance labels