stylescheme: add work around for broken selections
stylescheme: add work around for broken selections

Some selections in the wild have a background but no foreground set. For
those we must override the background because some GTK themes no longer
draw the foreground text (such as libadwaita).

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview