Controlling background fade when bringing up a dialog

Controlling background fade when bringing up a dialog

I'd like to bring up several modal dialog boxes using the UI dialog method.  I see that I can control the box background color and the text color and fonts by just styling the <div> ...<div> that dialog() uses so I don't think I'll need the theming.  But I don't see a way with the dialog method to control the amount of fadeout the main screen gets when the modal dialog appears.  Is there a simple way to control this without getting involved with themeroller and ui.themes.css? 

Thanks