#back-to-top-btn {
    position: fixed; right: 24px; bottom: 24px; z-index: 500;
    width: 46px; height: 46px; border: 0; border-radius: 50%;
    background: rgba(0,0,0,.55); color: #fff; font-size: 1.2rem;
    display: grid; place-items: center; cursor: pointer;
    box-shadow: 0 4px 16px rgba(0,0,0,.3);
    opacity: 0; pointer-events: none; transform: translateY(8px);
    transition: opacity .25s, transform .25s, background .2s;
}
#back-to-top-btn.show { opacity: 1; pointer-events: auto; transform: translateY(0); }
#back-to-top-btn:hover { background: rgba(0,0,0,.75); }