Hi Robert,
The jQuery CSS, and the conversion of the button from a native OS button is done by the switch "use jquery buttons" in the same place that you set the CSS for the button. I'm assuming you want the button changed to (the concept of) a JavaScript button. If not, just tick this off.
However in CSS terms you don't need to "remove" the jQuery class. You just need to set whatever it is you want to set in your class, and that will override the existing class. So if ui-button is setting the color to red, you can set it to blue, and yours will win. And so on.
cheers
Bruce