Comment 26 for bug 1599453

This should fix the scrollbars in Ambiance:

.vertical.scrollbar:hover:dir(ltr),
.vertical.scrollbar.dragging:dir(ltr) {
  margin-left:0;
  border-left:2px solid @dark_bg_color;
}

.scrollbar.dragging:not(.slider),
.scrollbar:hover:not(.slider),
.scrollbar:not(.slider) {
  background-color: @dark_bg_color;
}