What I advised (change <button to <a, and suppress ui-state-default class) came from directly looking at the link you cited (
http://jqueryui.com/demos/dialog/#modal-form): using Firebug, you may verify that this change matches your need,
at the resulting code stage.
This change concerns HTML code, not CSS contents!
In the other hand, I don't know which is your exact context of use: remember that I talked about the above link only.
I gave a general advice about HTML/CSS use, but if you're using jQuery UI components, I cannot say more, since I don't use jQuery UI, and I think you'd better posting in the jQuery UI forum.