public static function get_device_type_theme($devicetype = null) {
        global $CFG;
        if ($devicetype === null) {
            $devicetype = self::get_device_type();
        }
        $themevarname = self::get_device_type_cfg_var_name($devicetype);
        if (empty($CFG->$themevarname)) {
            return false;
        }
        return $CFG->$themevarname;
    }