Thanks for the help! When I open Plain10.msstyles in Windows Style Builder I navigate to Unknown, right? But which values belong to the scrollbar? I'm kind of confused.
Edit: Oh I think I get it. Since the scrollbar is not defined by your msstyles file but the whole theme package I'd probably have to change Windows 10.msstyles.
Okay so I have to live with that scrollbar or otherwise it'll be dark everywhere not only in the Start Menu. Isn't there a way to make it apply only to the Start Menu?