Arc is my favourite GTK theme. It has three official versions: a light one, a darker one and a dark one. Now a slightly modified version has been released with OS X styled window buttons. This version is not official, it’s totally independent from the original author. It is optimized for the Solus Desktop, but I tried it on my Gnome Shell and it work perfect to me (see screenshot below).





Download Arc GTK theme with OS X style buttons