Instance of |StylesPart| for this document. Creates an empty styles
        part if one is not present.

def _styles_part(self):
        """
        Instance of |StylesPart| for this document. Creates an empty styles
        part if one is not present.
        """
        try:
            return self.part_related_by(RT.STYLES)
        except KeyError:
            styles_part = StylesPart.default(self.package)
            self.relate_to(styles_part, RT.STYLES)
            return styles_part