Lookup value of *key* themeable

        If *key* not in themeable or value is None,
        return the *default* value.

def _default(self, key, default=None):
        """
        Lookup value of *key* themeable

        If *key* not in themeable or value is None,
        return the *default* value.
        """
        try:
            value = self.theme.themeables.property(key)
        except KeyError:
            value = None

        return value if value is not None else default