Source : http://askubuntu.com/questions/34214/how-do-i-disable-overlay-scrollbars

~/.gtkrc-2.0

style "default" {
  engine "murrine" {
    stepperstyle = 0
  }
}

~/.config/gtk-3.0/gtk.css

.scrollbar {
  -GtkScrollbar-has-backward-stepper: 1;
  -GtkScrollbar-has-forward-stepper: 1;
}