code: public Element toggleClass(String className) {
          Validate.notNull(className);
  
          Set<String> classes = classNames();
          if (classes.contains(className))
              classes.remove(className);
          else
              classes.add(className);
          classNames(classes);
  
          return this;
      }
docs: Toggle a class name on this element's {@code class} attribute: if present, remove it; otherwise add it.
  @param className class name to toggle
  @return this element