The following lines were added (+) and removed (-):
sudo vi /usr/share/themes/Mint-X/gtk-3.0/gtk-widgets.cssLook for .scrollbar { background-clip: padding-box; -GtkRange-trough-border: 2; -GtkScrollbar-has-backward-stepper: 0; -GtkScrollbar-has-forward-stepper: 0; -GtkRange-slider-width: 10; -GtkScrollbar-min-slider-length: 30; -GtkRange-stepper-spacing: 0; -GtkRange-trough-under-steppers: 1; }Change -GtkRange-slider-width: 10; to something bigger.