"... it seems that with every minor update to GTK3, themes get broken. I experienced this myself trying find a GTK3 theme that worked well with SpaceFM – most of the themes are broken in GTK 3.4 thru 3.6 (you can see the warnings when running SpaceFM in a terminal, and functions will be broken in the app). Thus the better-maintained themes (such as Clearlooks-Phenix and DarkMint) will have different versions for every minor version of GTK. The less well-maintained ones will simply remain broken.
Theme development is a tedious and difficult task, and for the GTK devs to be so careless in breaking their API at every turn disrespects the many hours people put into making themes for it. Yet as I read some of the GNOME developer comments below, I was given to believe that this breakage stems from a Microsoft-like climate of preventing users from customizing their systems, and deliberately breaking the work of others so that your ‘brand’ is the best. Anytime I hear the word ‘brand’ being used in Linux, I know something valuable is being poisoned.