public List<String> getThemeStyleSheets() {
   final List<String> result;
   if (contentGroup.getStylesheets().isEmpty()) {
     result = null;
   } else {
     result = contentGroup.getStylesheets();
   }
   return result;
 }
 public void setPreviewStyleSheets(List<String> previewStyleSheets) {
   contentGroup.getStylesheets().clear();
   contentGroup.getStylesheets().addAll(previewStyleSheets);
   contentGroup.applyCss();
 }