body { background: url(http://www.newsimg.cn/politics/2017qyh/img/banner.jpg) 50% 30px no-repeat;}
    body.bimu { background: url(http://www.newsimg.cn/politics/2017qyh/img/banner_bimu.jpg) 50% 30px no-repeat;}
    .nav li{
        width:250px;
    }
    .nav li.on,.nav li:hover{
        background-size: 100% 100%;
    }
	@media screen and (max-width: 767px){
        body {background: none;}
        body.bimu {background: none;}
        .banner{height:auto;}
        .nav li {
            width: 25%;
            height: 40px;
            line-height: 40px;
            margin-right: 0;
            font-size: 16px;
        }
    }