#back-to-top {
    position: fixed;
    bottom: 30px;
    z-index: 999999;

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

    width: var(--btt-size);
    height: var(--btt-size);

    font-size: calc(var(--btt-size) * 0.4);
    line-height: 1;

    color: #fff;
    border: none;
    border-radius: 50%;
    cursor: pointer;

    padding: 0;
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.3s ease;
}

.btt-right { right: 30px; }
.btt-left { left: 30px; }

#back-to-top:hover {
    opacity: 0.85;
}