Use dark wallpaper in dark mode when available

[danirabbit]

Problem

We currently only support a single wallpaper, but in dark mode folks might want to use a darker wallpaper that is better for dark environments, easier on the eyes, less contrasty, etc.

Proposal

There is a picture-uri-dark key in org.gnome.desktop.background. We should use that (if it isn't empty) when the system is in dark mode.

Currently use use picture-uri here:

gala/src/Background/BackgroundSource.vala

Line 25 in dd98aef

"picture-uri",

gala/src/AccentColor/AccentColorManager.vala

Line 75 in dd98aef

background_settings.changed["picture-uri"].connect (update_accent_color);

Prior Art (Optional)

No response