A GTK+ theme, such as a theme I made, requires GTK+ library to be installed before it can be used. Like an application, a GTK+ theme is designed for specific version of GTK+ libraries. For example, zonColor theme pack was designed for both GTK+ 2 and GTK+ 3, specifically GTK 2 ver. 2.20 and/or GTK 3 ver 3.4 as minimum requirements.
A GTK theme used in GNOME image viewer app (EOG) displaying GTK+ logo
So, the question is: How to find out the GTK version running in my OS?
There is no specific tool or command that can be used to answer that question, but you can try Terminal commands below, for both GTK2-based desktop (such as Ubuntu 10.04) and GTK3-based desktop (such as Ubuntu 12.04).