code: def default_font_toggled(self, settings, key, user_data):
          
          font_name = None
          if settings.get_boolean(key):
              gio_settings = Gio.Settings('org.gnome.desktop.interface')
              font_name = gio_settings.get_string('monospace-font-name')
          else:
              font_name = self.settings.styleFont.get_string('style')
          if not font_name:
              log.error("Error: unable to find font name (%s)", font_name)
              return
          font = Pango.FontDescription(font_name)
          if not font:
              log.error("Error: unable to load font (%s)", font_name)
              return
          for i in self.guake.notebook_manager.iter_terminals():
              i.set_font(font)
docs: If the gconf var use_default_font be changed, this method
          will be called and will change the font style to the gnome
          default or to the chosen font in style/font/style in all
          terminals open.