.back-to-top {
    position: fixed;
    bottom: 15px;
    right: 15px;
    text-decoration: none;
    color: #fff;
    background-color: rgba(235, 35, 35, 0.80);
    font-size: 10px;
    font-weight: bold;
    padding: 1em;
    display: none;
}

.back-to-top:hover {    
    background-color: rgba(135, 5, 5, 0.50);
}