code: private void addMainPanelHoverHandlers(Panel panel) {
  
          final CmsStyleVariable hoverPanel = new CmsStyleVariable(panel);
          hoverPanel.setValue(I_CmsLayoutBundle.INSTANCE.globalWidgetCss().openerNoHover());
          panel.addDomHandler(new MouseOverHandler() {
  
              public void onMouseOver(MouseOverEvent event) {
  
                  hoverPanel.setValue(I_CmsLayoutBundle.INSTANCE.globalWidgetCss().openerHover());
  
              }
          }, MouseOverEvent.getType());
          panel.addDomHandler(new MouseOutHandler() {
  
              public void onMouseOut(MouseOutEvent event) {
  
                  hoverPanel.setValue(I_CmsLayoutBundle.INSTANCE.globalWidgetCss().openerNoHover());
  
              }
          }, MouseOutEvent.getType());
      }
docs: Helper method for adding event handlers for a 'hover' effect to the main panel.<p>
  
  @param panel the main panel