The most common new feature that the themes are using that makes them not work with 3.5.1 is multiple fonts. If that's the only problem, it shouldn't be too hard to adapt them to work with 3.5.1.
In the .wps and .sbs files within the zip: Remove any line that starts with %Fl, and in any line starting with %V where the 5th number (a single digit) is greater than 1 to "1".
That won't necessarily make every theme work, and you might not like the results even if it does, but it might be worth trying.
I understand not wanting to use a daily build, but I've generally found that they're as stable as the official releases. And after 3.6 is released and all these themes work with it, it won't be long before new features are introduced and themes are created that don't work with 3.6...