code: def toggle_use_font_background_sensitivity(self, chk):
          
          self.get_widget('palette_16').set_sensitive(chk.get_active())
          self.get_widget('palette_17').set_sensitive(chk.get_active())
docs: If the user chooses to use the gnome default font
          configuration it means that he will not be able to use the
          font selector.