Implement freedesktop.org color-scheme preference

[salim-b]

The XDG Desktop Portal specification provides the org.freedesktop.appearance.color-scheme setting to indicate the system’s preferred color scheme. Respecting this would allow the Flatpak to automatically choose the proper theme variant (light or dark).

Currently, I have to manually override the theme variant like this to make the MediaInfo Flatpak open in dark mode:

flatpak override --user --env=GTK_THEME=Adwaita:dark net.mediaarea.MediaInfo

(I'm using GNOME, although the above should apply to all Linux desktop environments implementing the XDG Desktop Portal spec.)

More information

• GNOME blog post: Dark Style Preference
• GNOME initiative: Dark Style Preference