I think a cool Gorilla Enclave theme could be animals and plants from the Sonoran desert! The background could be a desert scene with cactus, like saguaros, barrel cactus, and prickly pears, like this:



The cacti could also be individual decors. And as far as the animal decors, there's a lot of possibilities.

Kangaroo rat:



Gila monster:



Horny lizard:



Javelina:



Mountain lion:



Ringtail:



Jackrabbit:



Sidewinder:



Kit fox:



Coyote:



Obviously, not all of these would make it into the game, and there's a lot of other possible animals and plants I didn't mention, but I hope this can show how much potential there is for a decor theme.