code: private function getButtonClasses()
      {
          $classes = $this->extraClass();
          if ($this->isGrouped()) {
              return $classes; //Do not return the below additional classes
          }
          $classes .= ' btn btn-default ss-ui-button';
          if ($this->getAttribute('target') != '_blank') {
              // Only add this class if this link is targeted inside the CMS. Any links targeted to a new browser
              // window/tab should not have this as the CMS JavaScript would hook to the 'onclick' event and load the
              // content via AJAX to the CMS, which could cause problems.
              $classes .= ' cms-panel-link';
          }
  
          return $classes;
      }
docs: Combines classes from $this->extraClass() with a couple of additional classes if they are
  applicable for this button.
  @return string