#btn{
    width: 40px;
    height: 40px;
    display: none;
    position: fixed;
    left: 50%;
    bottom: 40px;
    margin-left: 610px;
    background: url("https://images.cnblogs.com/cnblogs_com/abao0/975461/o_top_bg.png" )no-repeat left top;
}
#btn:hover{
    background: url("https://images.cnblogs.com/cnblogs_com/abao0/975461/o_top_bg.png" )no-repeat left -40px;
}
#back-to-top {
    background-color: #8bcc99;
    bottom: 10px;
    box-shadow: 0 0 6px #72a072;
    padding: 5px 10px;
    position: fixed;
    right: 50px;
    border-radius: 8px;
    opacity: 0.7;
}