I disagree. On my website I have a sidebar and a main content area. You are able to scroll through posts on the sidebar and through the content on the main content area. The sidebar is a dark color. Being able to use a thin sidebar and make the color of the sidebar dark to match the background makes the website look a lot better than having a clunky white sidebar on a dark background.
The user can still obviously see the sidebar and knows its a sidebar, it just works better with the design.
Ideally web browsers would use better defaults so you wouldn't have to do this. Firefox is good with this in my opinion: scrollbars are very subtle, don't affect content width, and adapt to the `color-scheme`.