The theme will also work on a number of other desktop environments
The Numix theme, one of the very first modern flat themes that combines a number of dark and light elements, with support for Gnome, Unity, XFCE and Openbox, has become shareware.The Numix developers were among the first to jump on the “flat” bandwagon, and it is absolutely gorgeous. Unfortunately, this comes with a cost. They didn't make any money off it, but put in a lot of hours of work.
“We've decided to commercialize the project a bit more and as a result we are making some of our stuff paid only,” reads the announcement on their Google+ account.
The team is actually responsible for a lot of themes and variants, not just the classic one. They split the project in several parts, and you will be able to purchase them from Ubuntu Software Center and a few other venues soon.
It's sad to see that such a promising project needs to go shareware in order to survive, but they do have to make a living somehow.