code: def only_pallets_theme(default=None):
      
  
      def decorator(f):
          @wraps(f)
          def wrapped(app, *args, **kwargs):
              if not app.config.is_pallets_theme:
                  return default
  
              return f(app, *args, **kwargs)
  
          return wrapped
  
      return decorator
docs: Create a decorator that calls a function only if the
      ``is_pallets_theme`` config is ``True``.
  
      Used to prevent Sphinx event callbacks from doing anything if the
      Pallets themes are installed but not used. ::
  
          @only_pallets_theme()
          def inject_value(app):
              ...
  
          app.connect("builder-inited", inject_value)
  
      :param default: Value to return if a Pallets theme is not in use.
      :return: A decorator.