Set a new theme to use or return current theme name

        :param theme_name: name of theme to use
        :returns: active theme name

def theme_use(self, theme_name=None):
        """
        Set a new theme to use or return current theme name

        :param theme_name: name of theme to use
        :returns: active theme name
        """
        if theme_name is not None:
            self.set_theme(theme_name)
        return ttk.Style.theme_use(self)