请帮忙看为什么不能实现背景颜色切换

Question

 <script>           (function($){               $.extend({                  'focus':(function(){                      $('li').mouseover(function(){                          $(this).css('background-color','red');                                          });                   });                  'blur':(function(){                     $('li').mouseout(function(){                          $(this).css('background-color','white');                      });                  });               });           })(jQuery);                      $(function(){               $.focus();               $.blur();           })       </script>