it will at least fail for `theme_matplotlib()`, `theme_seaborn()` and `theme_xkcd()` as these themes have a `__init__()`, which does not call `super(...).__init__()` and so `self.elements` will not be set. I think the correct fix would be to change these themes to call super and also change all themes to use self.elements as a slot for the callbacks -> would make overwriting `__radd__()` in all subclasses