Ubuntu 16.04 packs some modern software, including the latest stable (3.20 IS NOT stable, whatever the GNOME team may say) GIMP ToolKit. We had seen this coming, and because we had provided support for it in Fedora 22/23, it hasn't been much of a hassle to fix the remaining bugs.
However, the next-gen GNOME desktop will ship with GTK 3.20, which (almost) totally resets the way themes are rendered. According to this GNOME post, they've replaced many CSS classes with nodes, supposedly to improve speed and to allow further customization. Althought we've got most of the OS X and Windows 10 themes ported, it wasn't as easy as upgrading to GKT 3.18. Due to a lack of time, we have to redefine our priorities on the short and mid-run. Here is the expected roadmap for the next two or three months.