.back360{
    display:inline-block;
    width:40px;
    height:40px;
    position:fixed;
    bottom:10px;
    left:10px;
    z-index:1000;
    background:red;
    border-radius:60px;
    background:url(' ../img/back.png') no-repeat center center;
    background-size:100% 100%;
}