CSS on a popup

Okay, I am trying to set up pop up window that is attached to the current stylesheet I am using for my site. Everything works, except that the background image I have for my site I DONT want for the popup background. I don't want to create a separate stylesheet, I just want the background image to disappear without having to rewrite everything. I tried putting the body into a class, and that didn't work. Can you help?

You can define the background image in the header (between style tags) and not in the CSS file, leaving in the CSS file just the common definitions.

Hope it helps!