code: def on_gtk_theme_name_changed(self, combo):
          
          citer = combo.get_active_iter()
          if not citer:
              return
          theme_name = combo.get_model().get_value(citer, 0)
          self.settings.general.set_string('gtk-theme-name', theme_name)
          select_gtk_theme(self.settings)
docs: Set the `gtk_theme_name' property in dconf