It was reported on Release 108 (Safari 13.2, WebKit 15610.1.16.3) macOS 10.15.5
It appears the scrollbars aren't updating in while in dark mode. I have yet to confirm.
Created attachment 402309 [details]
With this reduced testcase, it seems the scrollbar color does not adapt to the background color in Dark mode with `color-scheme: light dark`.
If you remove `color-scheme: light dark`, the scrollbar always adapts to the background color.
When the page is in light mode, and the OS is dark mode, FrameView::recalculateScrollbarOverlayStyle() computes an overlay scrollbar style of ScrollbarOverlayStyleDefault, which I think means "use the OS default" which gives the wrong color.