Return the id of *style*, or |None| if it is the default style of
        *style_type*. Raises |ValueError| if style is not of *style_type*.

def _get_style_id_from_style(self, style, style_type):
        """
        Return the id of *style*, or |None| if it is the default style of
        *style_type*. Raises |ValueError| if style is not of *style_type*.
        """
        if style.type != style_type:
            raise ValueError(
                "assigned style is type %s, need type %s" %
                (style.type, style_type)
            )
        if style == self.default(style_type):
            return None
        return style.style_id