css基础应用2.0使用css设置块的各种属性，如div的长度和宽度，div中背景图片的设置，如何改变div中背景图片的

``` *{
            margin: 0;
            padding: 0;
        }
        body{
            background-color: gray;
        }
        dl{
            width: 325px;
            height: 40px;
            background-color:black;
            color: white;
            font:normal bold 25px '宋体';
            line-height: 40px;
            margin: 10px;
        }
        img{
            vertical-align: middle;
        }
        .title{
            width: 300px;
            height: 40px;
            background-color: white;
            background-position: 0px 0px;
            background-repeat: no-repeat;
            color: black;
            font: normal bold 15px '宋体';
            padding-left: 25px;
            line-height: 35px;
        }
        .content{
            width: 300px;
            height: 60px;
            line-height: 15px;
            background-color: white;
            border: none 1px transparent;
            border-bottom: dotted gray;
            color:gray;
            padding-left: 25px;

        }