a.back-to-top {
    position: fixed;
    top: 0px;
    z-index: 1000;
    display: none;
    width: 28px;
    height: 28px;
    background-color: #fff;     
    opacity: 0.6;
    filter: alpha(opacity=60);
}

    a.back-to-top:hover {
        opacity: 0.9;
        filter: alpha(opacity=90);
    }