Whatever theme you want!  Here are a few Birthday Party theme ideas:  Princess party, Dress Up party, Cooking party, Arts and Crafts party, Mermaid party, Garden party, Tea party, Fire Truck party, Cowboy party, Teddy Bears, Dance party, Horses, Doll party, Carnival party, Sports party, Dinosaurs party, Pirate party, Bug party, Trains party, Construction party, Military, Zoo party, Super Hero party, Adventure party, Spy party or Animal party.