Several apps have a very narrow scrollbar which I have trouble grabbing. This is the same in several themes I tried; I am currently using Breeze-Dark.
By modifying gtk style sheets, I got wider scrollbars in xfce4-terminal, thunar and geany.
https://forum.xfce.org/viewtopic.php?id=9152
https://forum.manjaro.org/t/enlarge-scroll-bar-width-how-to/47579
But this has no effect on featherpad, my preferred text editor. The featherpad developer explains (https://github.com/tsujan/FeatherPad/discussions/559) "The scrollbar width is determined by the active widget style, not by FeatherPad. No app should enforce such sizes; it's the job of Qt widget styles/themes to set them."
I got a clue from an error message when Featherpad is started from terminal:
$ featherpad
QApplication: invalid style override 'gtk' passed, ignoring it.
Available styles: bb10dark, bb10bright, cleanlooks, gtk2, cde, motif, plastique, Windows, Fusion
$ echo $QT_STYLE_OVERRIDE
gtk
$ export QT_STYLE_OVERRIDE=gtk2 ; featherpad Scripts/JPgt
The last command opens Featherpad with a nice, wide scrollbar :) so I added "export QT_STYLE_OVERRIDE=gtk2" to /etc/profile
At my low level of knowledge, this took some digging to resolve, so I hope this description will be useful to someone.