td.details-control {
    background: url('https://raw.githubusercontent.com/DataTables/DataTables/master/examples/resources/details_open.png') no-repeat center center;
    cursor: pointer;
}
tr.shown td.details-control {
    background: url('https://raw.githubusercontent.com/DataTables/DataTables/master/examples/resources/details_close.png') no-repeat center center;
}