theme: Get GTK+ colors from style context

Rather than using a single widget's style for GTK+ colors in themes,
use the style context parameter of the drawing functions for those
colors. Right now, a single style context is shared between frames,
but this will change to support different style variants.

https://bugzilla.gnome.org/show_bug.cgi?id=645355

src/ui/frames.c

@@ -2526,7 +2526,7 @@ meta_frames_set_window_background (MetaFrames   *frames,
       GdkVisual *visual;
 
       meta_color_spec_render (style->window_background_color,
-                              GTK_WIDGET (frames),
+                              frame->style,
                               &color);
 
       /* Set A in ARGB to window_background_alpha, if we have ARGB */