code: public static void addThemedCSSResource(String resource) {
  		Map<String, Object> viewMap = FacesContext.getCurrentInstance().getViewRoot().getViewMap();
  		@SuppressWarnings("unchecked")
  		List<String> resourceList = (List<String>) viewMap.get(THEME_RESOURCE_KEY);
  		if (null == resourceList) {
  			resourceList = new ArrayList<String>();
  			viewMap.put(THEME_RESOURCE_KEY, resourceList);
  		}
  
  		if (!resourceList.contains(resource)) {
  			resourceList.add(resource);
  		}
  	}
docs: Registers a themed CSS file that needs to be included in the header of the
  HTML file.
  
  @param resource The name of the resource file within the library folder.