button#back-to-top {
    position: fixed;
    bottom: 100px;
    right: 40px;
    width: 60px;
    height: 60px;
    transform: rotate(-90deg);
    background: #222222;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: 0.5s;
    cursor: pointer;
    opacity: 0;
    visibility: hidden;
    z-index: 2;
}

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

button#back-to-top:hover i {
    text-shadow: 0 0 4px #ccc;
}

button#back-to-top i {
    color: #fff;
    font-size: 28px;
}

.progress-ring {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

.progress-ring rect {
    transition: stroke-dashoffset 0.2s linear;
    stroke-linecap: square;
    stroke-width: 4;
    stroke: #ffffff;
    fill: none;
    stroke-dasharray: 360;
    stroke-dashoffset: 360;
}