.back-top {
    display: none;
    position: fixed;
    right: 30px;
    bottom: 160px;
    height: 74px;
    width: 74px;
    padding-top: 4px;
    font-size: 40px;
    text-align: center;
    line-height: 72px;
    border-radius: 50%;
    border: 1px #b3b3b3 solid;
}