Add scrollbar steppers again

[allo-]

From my local gtk.css:

.scrollbar {
  -GtkScrollbar-has-backward-stepper: true;
  -GtkScrollbar-has-forward-stepper: true;
/*  -GtkRange-stepper-size: 18px;
  -GtkRange-slider-width: 18px;*/
}

The 18px is commented out and it looks nice here, I think I found the snippet somewhere including these lines for controling the size. You may have a look if this should be changed.