This is off-topic, but I think you may know something about this issue.
In 2022, I was able to
somewhat style the scrollbar for the Dark Theme. It was not perfect at all, but provided good visibility for the handle. Nowadays I'm used to do just keyboard commands like 'home' in cover grid view, and just forget the scrollbar. Also, that only worked with the system default UI style.
With the recent addition of a more refined color customisation dialog, I've tried today two things:
1) try more combinations of colors in the dark theme for the currently customisable UI elements, but that didn't work well
2) I've tried runnning from source again and adding that code Kovid suggested in 2022, but the change wouldn't take. (that was
args = sys.argv[:1] args += ['-stylesheet', 'test1.qss'])
Some Qt based programs have a different scrollbar style in dark theme, where the handle has more like a light gray tone and the rest is black, so I wonder if that could somehow be tweaked in Calibre, with the recent Qt update.