#back-top {
     position: fixed;
     bottom: 10px;
     right: 5px;
     z-index: 99;
}
#back-top span {
     width: 150px;
     height: 174px;
     display: block;
     background:url(https://images.cnblogs.com/cnblogs_com/blogs/838370/galleries/2445781/o_250221085102_o_220716081416_5ad9ad7857b27.png)no-repeat center center;
}
#back-top a{outline:none}