I've never understood the concept of picking a name for it's nickname. I'm
Millie and my parents would never have dreamed of calling me
Millicent because what was the point - I'd have always been
Millie.
If you prefer
Theo then go with
Theo, I like both, in fact
Theodore is on my list but as a middle name. Nicknames are quite popular as given names nowadays anyway,
Hattie,
Jay,
Evie, and
Ben are all used without their longer forms. :)