I know about that being an option in Ubuntu MATE, at least. But AIUI that's about 'UI' as in the large-scale layout of the desktop environment, with the "Redmond" option giving you a 'traditional' taskbar and system notification tray. (You can definitely get the same thing going in Xfce, though it involves fiddling with the panels - and it's a default in Cinnamon and LXDE.) Widget style is its own thing, though.