The metacity theme format allows to use colors from the current
GTK+ theme in window decorations. Since GTK+ now gained support
for dark theme variants, window managers using that theme format
(metacity, mutter, compiz via gtk-window-decorator) should be able
to use colors from the correct variant; so in case a variant is
requested, export it in the _GTK_THEME_VARIANT property on
toplevel windows.
https://bugzilla.gnome.org/show_bug.cgi?id=645354
