I can't see the reason while the position of .ui-dialog-container must be set to relative within the script and not inside the theme css. When toggling this with firebug and I can't even see a visual difference. The problem is because it's set dynamically one can't override it with css.

line 102, rev 5143

		var uiDialogContainer = uiDialogContent.parent().addClass('ui-dialog-container').css({position: 'relative'});