#skip-top {
    position: fixed;
    right: 20px;
    bottom: 20px;
    height: 40px;
    width: 40px;
    border-radius: 50%;
    background: #aaaaaa;
    z-index: 9999;
    text-align: center;
    font-size: 3rem;
    line-height: 0;
    color: #FFFFFF;
    opacity: 1;
    transition: opacity .5s ease-in;
}

#skip-top.is-hidden {
    opacity: 0;
}