@media screen and (min-width: 576px) {
    .fixed-sm-top {
        position: fixed;
        top: 0;
        left: 0;
        right: 0;
    }
    .fixed-sm-bottom {
        position: fixed;
        bottom: 0;
        left: 0;
        right: 0;
    }
}

@media screen and (min-width: 768px) {
    .fixed-md-top {
        position: fixed;
        top: 0;
        left: 0;
        right: 0;
    }
    .fixed-md-bottom {
        position: fixed;
        bottom: 0;
        left: 0;
        right: 0;
    }
}