I dunno about phpNuke, but when I was using PostNuke, it's theme system sucked(my personal opinion only). It was easy, but there was far less freedom for variation, i.e. every theme still looked like every other postnuke site. I'm not sure how it is these days.
Since I started using and developing for GL back in Febuary of this year, the number of themes has grown from 4 to 29(as Dirk pointed out). That's pretty darn good actually, and bodes well for the system as a whole.
Also, taking a 1.3.5 theme and upgrading it to 1.3.6 capability is as simple as reading the documentation in the 1.3.6 release.
I also plan on making all the publicly available themes available on my site(geeksta.com) at some point in the near future. Once I've done that, I may consider "upgrading" other author's 1.3.5 themes if they don't have plans for it themselves, but that is probably a while off, given the time constraints I'm under these days.
Anyways, hang on if you don't see a theme you like, chances are, in the future, you probably will.