code: def base_style(self):
          
          basedOn = self.basedOn
          if basedOn is None:
              return None
          styles = self.getparent()
          base_style = styles.get_by_id(basedOn.val)
          if base_style is None:
              return None
          return base_style
docs: Sibling CT_Style element this style is based on or |None| if no base
          style or base style not found.