Using the “Dark Mode” theme, which is excellent btw, the scroll bar becomes very hard to see, and you have to spend time
looking for it every time. This problem is amplified when you use things like f.lux, where differentiating between different colors become a little harder.
The solution is to either make the color of the scroll bar customizable, or change the color of it all together.
This has been asked before, but the question was misunderstood. (LINK)
Ditto! Exactly! This is the only post on scroll bar I found. Love the dark theme, but almost impossible to find the button in the scroll bar. I’ve customized the colors in Appearance setting as much as I can; made the editor a little lighter, which helps to see the scroll bar, but button is still almost invisible.
Found an article in Google on editing size of scroll bar in Windows, and button size, and went into Registry and edited, which worked on some programs, but no effect in Scrivener.
Hope can add scroll bar button to Appearance options; bright red would be nice! 
Try exporting the current Theme, unzip the contents, modify the css, pack it back into zip, rename the extension and load the modified Theme back.
