code: public function merge($items, $addPath = false)
      {
          $themesPaths = $this->getThemesPaths();
          /* @var $themeCollection $this */
          $themeCollection = parent::merge($items);
  
          $themeCollection->requiredFields = $this->requiredFields;
          $themeCollection->exceptionOnInvalid = $this->exceptionOnInvalid;
  
          if ($addPath !== false) {
              $themesPaths[] = realpath($addPath);
          }
          foreach ($themesPaths as $path) {
              $themeCollection->themesFolders[$path] = $path;
          }
  
          return $themeCollection;
      }
docs: @param mixed          $items
  @param string|bool $addPath
  
  @return static