r2c/user/themes/r2c/css/resizer.css

[data-rz-handle] {
    flex: 0 0 6px;
    background-color: rgba(0, 0, 0, .75);
}

[data-rz-handle] div {
    width: 6px;
    background-color: rgba(0, 0, 0, .5);
}