Mate default theme has the scrollbar set pretty nice, but I still like to tweak it a bit. This is something I don't want to do everytime I switch themes. So here's a way to tweak the scrollbar that will work for all the gtk3 themes.
NOTE: This will change most, but not all windows. Example: No effect on Synaptic Package Manager.
Create a new file and name it "gtk.css".
Place this file in your home folder at the location /.config/gtk-3.0/gtk.css
And place this script in your newly created "gtk.css" file:
scrollbar slider {
/* Size of the slider */
min-width: 15px;
min-height: 15px;
border-radius: 17px;
/* Padding around the slider */
border: 2px solid transparent;
}
This is set to my liking (15,15,17,2), but you can play with all four of the "px" numbers until it's to your liking.