Figure out who you want to talk to in the department (they should have bios on the department page), email that person, and tell them that you’re really interested in their program, but despite your good grades, you’re worried that your liberal arts background will put you at a disadvantage. Ask them what you can do to help improve your chances of being accepted into their program – is there perhaps a few classes you should be taking in the meantime? Any other suggestions they might have?