You can affect system-wide scrollbar width with a custom gtk.css file placed in your ~/.config/gtk-3.0 directory.
It may be possible to isolate only the Evolution app, by sussing out its css class. I would need to log into Gnome and launch Evolution with GTK-Inspector to see if that can be located and whether or not it classifies the scrollbars. If it does not, then only a system-wide setting would affect it.