When the UI got updated in the past, it was reported as a bug and never fixed.
A side note on mobile the UI is the same for all site, as such the color are better suited for question with accepted answer
See there for example; http://meta.serverfault.com/a/8027/126632https://meta.serverfault.com/a/8027/126632
A report I did in the past too; SF layout / Answer coloringSF layout / Answer coloring