docs: def add_style_of_type(self, name, style_type, builtin):
          """
          Return a newly added `w:style` element having *name* and
          *style_type*. `w:style/@customStyle` is set based on the value of
          *builtin*.
          """
          style = self.add_style()
          style.type = style_type
          style.customStyle = None if builtin else True
          style.styleId = styleId_from_name(name)
          style.name_val = name
          return style