The new non-NSScroller scrollbars' arrows remain the same dark shade of grey when they are in a background window. To reproduce: open the URL in TOT and switch to a different window. Notice how the shade of the scroll thumb changes, but the arrows stay the same. In Safari 3.1 the arrows change to a much lighter shade.
Note that Finder has the same problem.
<rdar://problem/7073971>
I think it is below 'WebKit' level and scrollbar are from 'macOS', we don't get this issue anymore. Marking this as 'RESOLVED CONFIGURATION CHANGED', please reopen, if it is reproducible.