The dress codes for the signature dining restaurants are for the most part just a suggestion. They only things that are not permitted are swimwear, tanktops,mens' hats, torn or tattered clothing, and t-shirts with offensive language or images. Oddly, the only two signature restaurants that do not list a dress code on the Disney website are The Brown Derby and Lecellier.
Here it is directly from the Disney web site and exactly the same for each signature restaurant that has a dress code listed:
Guest Policies
This experience takes place in a non-smoking environment.
This location can accommodate most special dietary needs. Advance notice may be required and cancellation fees may apply. Please indicate any special dietary need at time of reservation booking. Learn more about how we can help accommodate special dietary needs.
Dress Code Guidelines:
Men: Khakis, slacks, jeans, dress shorts, collared shirts. Sport coats are optional.
Ladies: Capris, skirts, dresses, jeans, dress shorts.
Not permitted in dining room: Tank tops, swimwear, hats for gentleman, cut offs, or torn clothing. While T-shirts are now allowed, the policy remains that T-shirts with offensive language or graphics are not acceptable.
Hope that helps!!