On MacOS, all scrollbars inherit their colours from the operating system light/versus dark colour palette.
For dark mode you can see examples of css used to modify the scrollbars on each platform here:
https://github.com/Sigil-Ebook/Sigil...rce_Files/dark
Check out examples of css in win_dark_scrollbar.css, lin_dark_scrollbar.css and mac_dark_scrollbar.css
You can add custom css that Preview loads and even cycle among them.