.btn.fixed {
    position: fixed;
    left: 0;
    right: 0;
    bottom: 0;
    margin-inline: auto;
    z-index: 9999;
}
@media screen and (max-width: 768px) {
    .btn.fixed {
        width: 90%;
        bottom: 10px;
        filter: drop-shadow(0 1vw 1vw rgba(0, 0, 0, 0.3));
    }
}