All I have found so far with suggestions have
been user-specific, via ~/.config/gtk-3.0/*.css.
Firefox uses the defaults set in gtk3, so that is the solution.
My interest is global, so that all users
automatically have useable scrollbars. Has anyone here found any that work at all?
The scrollbar default widths are set in the source in gtk/theme/Adwaita/gtk-contained.css. So if you want a different default, you'll have to build gtk3 yourself.
I set these and other options in gtk.css and settings.ini to get a more gtk2 like experience .. see https://ray-v.github.io/mozbuild/building-firefox-seamonkey-thunderbird-usin... for details of what you'll need to change.