[onion2k]

But I can almost guarantee the larger group of non-technical users will have never thought about the styling of their scrollbars for more than a few seconds.

I think you're saying that this means it doesn't matter, but it actually increases the importance of respecting the users choices. Anyone who has changed their scrollbar has done so because it's important enough to them to bother. They've put effort in. That means there is far more likely to be a good reason not to override their preferred styling.