Apply this theme, then apply additional modifications in order.

        Subclasses that override this method should make sure that
        the base class method is called.

def apply(self, ax):
        """
        Apply this theme, then apply additional modifications in order.

        Subclasses that override this method should make sure that
        the base class method is called.
        """
        for th in self.themeables.values():
            th.apply(ax)