#backToTop {
    position: fixed;
    bottom: 25px;
    right: 25px;
    width: 44px;
    height: 44px;
    border: none;
    border-radius: 50%;

    background-image: linear-gradient(rgb(96, 107, 133) 0%, rgb(86, 98, 129) 13%, rgb(59, 72, 104) 78%, rgb(57, 69, 101) 91%, rgb(60, 72, 103) 100%);
    color: #fff;

    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;

    opacity: 0;
    visibility: hidden;

    /* départ plus haut et plus à droite */
    transform: translate(120px, -800px) rotate(-8deg);

    transition:
        transform .8s cubic-bezier(.22,1.2,.36,1),
        opacity .5s ease,
        visibility .5s;

    z-index: 999;
}

#backToTop.show {
    opacity: 1;
    visibility: visible;

    transform: translate(0,0) rotate(0);
}

#backToTop:hover {
    transform: translateY(-4px);
}