'Sibling CT_Style element this style is based on or |None| if no base
style or base style not found.'

@property
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
