<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def only_pallets_theme(default=None):
 """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. """


    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