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

@property
def _styles_part(self):

    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
