th.sortable {
    cursor: pointer;
    position: relative;
}

th.sortable::after {
    content: '↑↓';
    margin-left: 5px;
    opacity: .4;
}

th.sortable_asc::after {
    content: '↑';
    opacity: 1;
}

th.sortable_desc::after {
    content: '↓';
    opacity: 1;
}