How to enable modal dialog lightbox effect without loading jQueryUI theme CSS?

How to enable modal dialog lightbox effect without loading jQueryUI theme CSS?

I'm using jQueryUI in a page to render a DIV as a modal dialog, but it's not appearing with the same lightbox/modal presentation as the demo at http://jqueryui.com/demos/dialog/#modal-form (i.e. page around the dialog should be dimmed + not clickable).

I'm not loading any jQueryUI theme/CSS files, which I suspect is the problem, but I don't need any of it for this (until now), so what is the bare minimum I need to do (e.g. specific CSS classes to add to my own CSS) to correctly obtain this modal/lightbox presentation effect? (I can't see anything relevant in the "Theming" tab)