Alan,
I can't tell you the specifics of what CSS to enter, but if you can find a local programmer it should not be too tricky for them to use a CSS investigation tool such as Chrome's "Inspect element" or the Firefox "Firebug" plugin, to find out which CSS style is affecting the background color and to create a more specific CSS selector to override it. Show your programmer this message if you like. It may help to speed up the investigation.
Another thing you should check is the "Body styles" setting on the following page:
- Administration -> Site administration -> Plugins -> Activity modules -> HotPot
If you select "Background color and image", then your HotPot colors will have precedence over the default colors in the theme that your Moodle site is using (see attached screenshot)
HTH,
Gordon
