docs: 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