Set theme for this docuemnt.

        This method sets theme's js/css files and headers on this document.

        :arg ModuleType theme: a module which has ``js_files``, ``css_files``,
            ``headers``, and ``extended_classes``. see ``wdom.themes``
            directory actual theme module structures.

def register_theme(self, theme: ModuleType) -> None:
        """Set theme for this docuemnt.

        This method sets theme's js/css files and headers on this document.

        :arg ModuleType theme: a module which has ``js_files``, ``css_files``,
            ``headers``, and ``extended_classes``. see ``wdom.themes``
            directory actual theme module structures.
        """
        if not hasattr(theme, 'css_files'):
            raise ValueError('theme module must include `css_files`.')
        for css in getattr(theme, 'css_files', []):
            self.add_cssfile(css)
        for js in getattr(theme, 'js_files', []):
            self.add_jsfile(js)
        for header in getattr(theme, 'headers', []):
            self.add_header(header)
        for cls in getattr(theme, 'extended_classes', []):
            self.defaultView.customElements.define(cls)