css怎样让背景充满整个屏幕?

<!DOCTYPE html>
<html>
<head>
        <meta charset="UTF-8">
        <title>炫酷时钟</title>
        <style type="text/css">
                body{
                        height: 100%;color: #51555c;
                        background: url("./img/bg1.png") no-repeat;
                                        /* 背景图垂直、水平均居中 */
                        background-position: center center;
                        
                        /* 背景图不平铺 */
                        background-repeat: no-repeat;
                        
                        /* 当内容高度大于图片高度时,背景图像的位置相对于viewport固定 */
                        background-attachment: fixed;
                        
                        /* 让背景图基于容器大小伸缩 */
                        background-size: cover;
                        
                        /* 设置背景颜色,背景图加载过程中会显示背景色 */
                        background-color: #464646;
                }
                img{
                        display: inline-block;
                        width: 28px;
                        height: 50px;
                        margin:0 4px;
                }
                .main_demo{
                        width: 910px;
                        min-height: 600px;
                        margin:30px auto 0 auto;
                }
                .main_demo h2{
                        text-align: center;
                        padding: 10px;
                        font-size: 40px;
                        color: wheat;
                }
                .clock{
                        width: 500px;
                        padding: 40px;
                        margin:2px auto;
                }

                .clock_right{
                        text-align: right;
                        margin:4px;
                }
        </style>
</head>
<body>
        <div class="main_demo">
                <div align="center">
                        <h2>Js 炫酷时钟</h2>
                </div>
                <div class="clock">
                        <!--10点-->
                        <img src="" />
                        <img src="" />
                        :
                        <!--40分-->
                        <img src="" />
                        <img src="" />
                        :
                        <!--*秒-->
                        <img src="" />
                        <img src="" />
                        <br />
                        <div class="clock_right">
                                <!--2017年-->
                                <img src="" />
                                <img src="" />
                                <img src="" />
                                <img src="" />
                                :
                                <!--09月-->
                                <img src="" />
                                <img src="" />
                                :
                                <!--27日-->
                                <img src="" />
                                <img src="" />
                        </div>
                </div>
        </div>

</body>
<script type="text/javascript">
        var imgs = document.getElementsByTagName('img');
        setInterval(getTime,1000);

        function getTime(){
                var _date = new Date();
                var year = _date.getFullYear();
                var month = _date.getMonth()+1;
                var day = _date.getDate();
                var hour = _date.getHours();
                var minutes = _date.getMinutes();
                var second = _date.getSeconds();
                var newDate = addZero(hour) + "" + addZero(minutes)+ "" + addZero(second)+""+addZero(year) + "" + addZero(month)+ "" + addZero(day)
                        ;
                console.log(newDate.length);
                for(var i = 0;i < newDate.length;i++){
                        imgs[i].src = 'img/time_' + newDate[i] + '.png';
                }
        }

        getTime();

        function addZero(num){
                if(num < 10){
                        num = "0" + num;
                }
                return num
        }
</script>
</html>