public function getTheme($identifier)
    {
        $themes = $this->getAvailableThemes();
        if ( !isset($themes[$identifier]) ) {
            $themeIds = array();
            foreach ($themes as $key => $value) {
                $themeIds[] = $key;
            }
            throw new \Exception(sprintf('Theme "%s" does not exist. Possible values are [%s]', $identifier, implode(', ', $themeIds)), 1);
        }
        return $themes[$identifier];
    }