#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};