public function themeExists($themeId) {
        // while this seem to be obsolete in most cases
        // it can be useful when the function is called from the outside
        $prefixedId = $this->getPrefixedThemeId($themeId);
        $map = $this->registry->getMap();
        if(empty($map['items']['available'])) {
            return false;
        }
        foreach($map['items']['available'] as $theme) {
            if($theme['id'] === $prefixedId) {
                return true;
            }
        }
        return false;
    }