add to .config/gtk-3.0/gtk.css

[http://dummy.us.eu.org/robert (Robert)]

scrollbar slider {
    /* size of the slider */
    min-width: 20px;
    has-backward-stepper: 1;
    has-forward-stepper: 1;
    /*min-height: 20px; */
    /* border-radius: 22px; */
    /* padding around the slider */
    /*border: 5px solid transparent; */
}