What is the dress code when going to the Vatican?

I am going to be touring the Sistine Chapel and St. Peters Basilica with my study abroad program. Is it appropriate to wear a dress from Forever 21 with leggings that cover my legs to mid calf? Or should I go out and buy another longer dress or pants. Also we cannot check into our hotel room until after these tours and we are traveling by train, so I want to be comfortable because we will be doing a lot of walking.