public function getThemeBootstrapVersion($themeName = null)
    {
        if (null === $themeName) {
            if (null !== $this->bootstrapVersion) {
                return $this->bootstrapVersion;
            }

            $themeName = $this->getActiveThemeBackend()->getThemeName();
        }

        $this->bootstrapVersion = $this->container->getParameter('red_kite_cms.bootstrap_version');

        if ( ! $this->container->hasParameter('red_kite_labs_theme_engine.bootstrap_themes')) {
            return $this->bootstrapVersion;
        }

        $themes = $this->container->getParameter('red_kite_labs_theme_engine.bootstrap_themes');
        if (array_key_exists($themeName, $themes)) {
            $this->bootstrapVersion = $themes[$themeName];
        }

        return $this->bootstrapVersion;
    }