do you have to make a game about frogs and can we use unity

From the FAQ:

"Do I have to follow the theme? No, but well… When in Rome~" and "What engine can I use? You can use any engine or framework you like..."

So no, it doesn't HAVE to be about frogs, but it is encouraged. It will HAVE to follow whatever the limitation is though. And yes you can use unity.

Hope that helps.