frames: Force on dark theme for all apps if the user requested it

If the user requested a dark theme for all apps through GNOME Tweak
Tool, go ahead and force it for all apps, not only GTK+3 apps.

Thanks to MaTachi on reddit who suggested the idea:

http://www.reddit.com/r/linux/comments/2r1zwj/linus_i_dont_know_who_thought_it_was_a_good_idea/cnc10ui