The styles from /usr/share/themes/*/gtk-3.20/gtk.css need to be overridden. Because the GTK-tards changed yet once again how the system works for no good reason, many previous solutions to the problem stopped working.
The CSS class is now scrollbar, was previously .scrollbar. -GtkRange-slider-width and -GtkRange-stepper-size were apparently removed, use min-width/min-height on the widget element instead.
My content of ~/.config/gtk-3.0/gtk.css:
scrollbar, scrollbar button, scrollbar slider {
-GtkScrollbar-has-backward-stepper: true;
-GtkScrollbar-has-forward-stepper: true;
min-width: 25px;
min-height: 25px;
border: 1px solid #5d9797;
border-radius: 0;
}
scrollbar slider {
background: theme_bg_color;
}
scrollbar slider:hover {
border-color: #418bd4;
}
Looks like this; close enough for me.
