#back_top div{
    position: absolute;
    margin: auto;
    right: 0px;
    left: 0px;
    z-index: 999;
}
#back_top{
    /*background-color: #dddddd;
    height: 38px;
    width: 38px;
    border-radius: 3px;*/
    display: block;
    cursor: pointer;
    position: fixed;
    right: 5px;
    bottom: 200px;
    display: none;
    z-index: 999;
}