#back-to-top {
    position: fixed;
    bottom: 3em;
    right: 3em;
    background-color: rgba(255, 0, 0, .9);
    color: #313943;
    border: none;
    border-radius: 5px;
    padding: 1em;
    text-transform: uppercase;
    cursor: pointer;
    font-weight: 700;
    box-shadow: 0 0 2em 0 rgba(0, 0, 0, .25);
    transition: all .3s ease-in-out;
    display: inline-block;
    opacity: 0;
    text-decoration: none;
    font-size: .75em;
}

    #back-to-top:hover {
       color: #fff;
       background-color: rgba(255, 0, 0, .9);
       padding: 1em 3em;
    }

#back-to-top.visible {
    opacity: 1;
}