Here's the portion of the FAQ about proper clothing to wear to the theme parks:
Q. What is the best way to dress for a day at the Parks? Is there any clothing that isn't permissible?
A. The Parks are a casual, family-oriented environment. We suggest you dress comfortably, wear good walking shoes and check the local weather report before you leave for the Parks.
It is an important part of the Disney experience that the Parks are family-friendly. In that spirit, we ask you to use your discretion and common sense regarding your attire in the Parks. Attire that is not appropriate for the Theme Parks (and which may result in refusal of admittance) includes, but is not limited to:
Adult costumes or clothing that can be viewed as an actual character
Masks (unless you are dressing up for a particular event)
Easily visible obscene or objectionable language or graphics
Excessively torn clothing
Clothing with objectionable material
Clothing which, by nature, exposes excessive portions of the skin that may be viewed as inappropriate for a family environment
Objectionable tattoos
An adult or older Guest is allowed into the Park if their hair and/or make-up has been done to resemble a Princess or character (e.g., after a Bibbidi Bobbidi Boutique makeover) provided the Guest is not also wearing a costume or clothed to look like the character
Hope this helps!