theme: Add the .background style class back to frames

GtkWindow added the BACKGROUND style class to all windows, which the
CSS file selects on to set a background color for all windows. Without
this, the background color becomes undefined, and thus window frames
look like they have "glitchy" graphics.

https://bugzilla.gnome.org/show_bug.cgi?id=690317

src/ui/theme.c

@@ -5454,6 +5454,7 @@ meta_theme_create_style_context (GdkScreen   *screen,
   style = gtk_style_context_new ();
   path = gtk_widget_path_new ();
   gtk_widget_path_append_type (path, META_TYPE_FRAMES);
+  gtk_widget_path_iter_add_class (path, -1, GTK_STYLE_CLASS_BACKGROUND);
   gtk_style_context_set_path (style, path);
   gtk_widget_path_unref (path);