Set prefer-dark color scheme for GNOME apps

Several GNOME apps show up in light mode by default even though all of the rest of the config is set to make GTK apps appear as dark. This change patches Sway and Wayfire to specify 'prefer-dark' for GNOME apps to improve consistency here.

(test with e.g. Apostrophe, Tuba, Activity Monitor which will open in light mode by default, but switch to dark mode when this setting is applied)