a.asc:after, a.desc:after {
    content: '';
    left: 3px;
    display: inline-block;
    width: 0;
    height: 0;
    border: solid 5px transparent;
    margin: 4px 4px 2px 4px;
    background: transparent;
}

a.asc:after {
    border-bottom: solid 7px #212529;
    border-top-width: 0;
}

a.desc:after {
    border-top: solid 7px #212529;
    border-bottom-width: 0;
}