[Edited after reading excellent comments!]
@Sharon & @StevenR : Under PureOS 9, the default scrollbar width in KDE applications is wider than in GNOME applications. In Okular, you can even turn the scrollbars off and still scroll because the entire page becomes a full-width scrollbar that you can scroll with a scroll wheel or click and drag.
When mouse scroll wheels, trackballs, the IBM Thinkpad joy button, and trackpads arrived, scrolling became possible without dragging on a scrollbar. Someone needs to do a study of what was going on in GUI design to see when scrollbars started to get narrower. I first noticed narrow scrollbars on Apple iPods, where the scrollbar just gave visual feedback. There was no mouse or touch capability. You only had the scroll wheel. Then, on Apple iOS touchscreens (iPhone & iPad), your finger became the mouse and you swipe to scroll. The scroll bars are invisible until you start swiping, then they appear and serve as visual feedback, then they disappear after the scrolling stops. No doubt some people don’t like that GUI design, and with Apple you can’t change it. The beauty of GNU/Linux is that you can change anything. Often the trick is just finding out where the config files are for whatever you want to configure.
You can click on and use skinny scrollbars, but doing that is so much slower than spinning a scroll wheel unless you have a long web page, text file, PDF or other document. In that case, scrolling with a scroll wheel takes longer than simply clicking on the scrollbar. It depends on your use case. With a scroll wheel you have a much larger target: the entire window of your application. If you are sequentially reading something, and scrolling slowly as you read, you usually don’t need to click the scrollbar. If you’re editing code, you do often need to go back and forth in files, so then the scrollbar might be faster. We also have the Home and End keys (Ctrl-Home and Ctrl-End in Kate) to instantly go to the top and bottom of the page, faster than any clicking.
The Logitech M325 Wireless Mouse is $13, but its scrollwheel doesn’t have as nice a feel as the $16 Logitech M185 Wireless Mouse, whose scroll wheel has detents that make for more precise control. So far all the Logitech stuff works out of the box with PureOS on a Librem 15v3, Librem 15v4, and Librem Mini v1.
By the way, @amarok & @tracy , I tested the ~/.config/gtk-3.0/gtk.css file, and it doesn’t affect KDE programs like KMyMoney, Kate, and Okular. Cool to know about that customization option, though.