GNOME 3.12 Can Be Enhanced with the Beautiful Zukitwo Theme
The Zukitwo theme was one of the first to get GNOME 3.12 support, shortly after the launch of the new desktop environment. This theme only works with GNOME Shell 3.12 and GTK+ 3.12, which means that it might create problems on other systems.
According to the changelog, the progress bar has been corrected and the button border is now better focused. This should make the buttons much more visible.