The reality is that if you have scrolling in the page (like a column layout for example), you kinda have to customize the scrollbars. If only to not break your layout as scrollbars are completely different everywhere, hidden, different widths, some ugly, some nice, no reliable way to detect the situation, it's a mess.
I would opt for more options: give us better tools to style (around) scrollbars, give us the tools to honor what the user prefers, to know when and when not to blend it in with the UI, to increase contrast, or to just leave it alone.
Just not customizing is not a realistic solution for a lot of situations.