If you click on the link
HERE, you'll be taken to a page on the WDW website that gives the answers to frequently asked questions about the theme parks, including guidelines for dress. Here's the relevant part:
What is the best way to dress for a day at the parks? Is there any clothing that isn't permissible?
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.
Ensuring that the parks are family friendly is an important part of
the Disney experience. In that spirit, we ask you to use your
discretion and common sense. 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 representative of an actual Disney character
- Masks (unless you are dressing up for a particular event)
- Clothing with objectionable material, including obscene language or graphics
- Excessively torn clothing
- Clothing which, by nature, exposes excessive portions of the skin that may be viewed as inappropriate for a family environment
- Objectionable tattoos
A Guest is allowed into the parks if her or his hair (or make-up, if
applicable) has been made to resemble a Disney Princess or character
(for example, after a Bibbidi Bobbidi Boutique makeover) provided the
Guest is not also wearing a costume or clothed to look like the
character.
In general, you can expect to see people wearing shorts, t-shirts, and sneakers at the parks if the weather is warm, and jeans if the weather is cooler. Just be comfortable and have fun.