Return a newly added `w:style` element having *name* and
        *style_type*. `w:style/@customStyle` is set based on the value of
        *builtin*.

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