You are looking for new themes or a different Social Platform?
I am sorry - not sure what the difference is!!!
You're not looking for a different platform, Anette. Unless you are thinking of leaving Ning, and I can tell by your question you are not.
Bernard has offered a solution, you might also be interested to know that Jen does Ning customization. She's been around on Ning longer than anyone I know, and does superior work.
If you don't like them then you're probably looking at some custom design work. This group might help you.