jquery中怎么修改css样式

.div1{

      width:100px;

      height:100px;

      background:#00ff00;

      }

.div2{

       width:100px;

       height:100px;

       background:#ff0000;

      }#Btndiv{

         width:100px;

         height:100px;

         border: 1px solid #ccc;

         margin-bottom: 10px;

         }