Set JQuery UI modal dialog overlay background color

That is in this css element:

.ui-widget-overlay {
   background: #AAA url(images/ui-bg_flat_0_aaaaaa_40x100.png) 50% 50% repeat-x;
   opacity: .30;
   filter: Alpha(Opacity=30);
}

it is on line 294 of: jquery-ui-1.8.11.custom.css