#back-top {
    position: fixed;
    bottom: 10px;
    right: 5px;
    z-index: 99;
}
#back-top span {
    width: 50px;
    height: 64px;
    display: block;
    background: url(https://images.cnblogs.com/cnblogs_com/seanshao/855033/o_rocket.png) no-repeat center center;
}
#back-top a {
    outline: none;
}