When adding the jQuery UI scripts to the page, set the include CSS value to false and add it in the page header manually. This will allow the jQuery UI styles to be loaded after your site specific files and still allow you to override some of the styles classes and still use the "built-in" styles.

I believe all I did was add the classes below to my stylesheet and it worked as expected:

.ModalBackground, .modalBackground { z-index: 20100 !important; background-color: #262B33; filter: alpha(opacity=70); opacity: 0.7; }