#back-to-top {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 50px;
    height: 50px;
    text-align: center;
    position: fixed;
    transition: background-color .3s,
    opacity .5s, visibility .5s;
    opacity: 0;
    visibility: hidden;
    z-index: 1000;
}
#back-to-top:hover {
    cursor: pointer;
}
#back-to-top.show {
    opacity: 1;
    visibility: visible;
}