In the Mortal Enguines series Theo is one of the main characters in Infernal Devises, and Darkling Plain. He marries Tom and Hester's daughter Wren. Called Theo Ngoni.
I love the nickname Theo - for a girl. I think it makes an absolutely darling nickname for something like Theodora or Theodosia. It's got spunk and it's vintage, and I like the way it has an almost masculine feel but can still work on a girl. Then again, I like it on a boy as well, so that's that.
Theo sounds like a boys name to me! I've never known a girl called Theo! Fi-Fi or Thea is a girly spin on the name! My son is called Theo and I'd never dream of calling a girl Theo. Thea / Theo - very obvious which is a boyish name and which is girly.