[Feature request] Allow changing background color on "contain" mode wallpapers

[repparw]

https://github.com/hyprwm/hyprpaper/blob/6f4ba431638e44478417b9f524fb1771e5eda83c/src/Hyprpaper.cpp#L561-L565

I think this is the relevant code snippet, when using contain mode for wallpapers, (e.g different aspect ratio photo) the rest of the background gets drawn as #000000, would like to change this to a shade of grey in my color palette