I think Theo goes well with:
Amelie, Amy, Ariel, Alexis, Annabel, Bryony, Celeste, Claudia, Clarissa, Camilla, Darcey, Elina, Eloise, Erica, Erin, Felicity, Faith, Grace, Giselle, Hannah, Hope, Harriet, Imogen, Iris, Juliet, Jasmine, Katy, Leonie, Lois, Lara, Maia, Melissa, Miranda, Miriam, Natalie, Natasha, Naomi, Orla, Phoebe, Polly, Rosie, Robyn, Rowena, Romilly, Sadie, Sylvia, Saskia, Sorrel, Verity, Vanessa, Xanthe, Zara
Angus, Aidan, Aaron, Anton, Benedict, Christian, Caleb, Callum, Dominic, Elliot, Ezra, Ethan, Fergus, Finley, Freddie, George, Harvey, Harry, Ivan, Jeremy, Jonty, Jamie, Kieran, Logan, Micah, Milo, Monty, Nicholas, Owen, Patrick, Rory, Robin, Rowan, Solomon, Seth, Victor, Vincent, Xavier, Zachary