.btn-xx {
    position: fixed;
    right: 20px;
    z-index: 9999;
    padding: 10px 15px;
    font-size: 20px;
    background-color: #333;
    color: #fff;
    border: none;
    border-radius: 50%;
    cursor: pointer;
}
#btn-top {
    bottom: 80px;
}
#btn-bottom {
    bottom: 20px;
}
.scroll-btn {
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.4s, visibility 0.4s;
}
.scroll-btn.show {
    opacity: 1;
    visibility: visible;
}