Opera's transparent scrollbar backgrounds

Hi

I'm working on a webpage (hosting still to be determined!) that has a CSS faked IFrame (ie: scrolling content in a section of the page). Everything works fine, except that the background image shows through the scrollbar, not the thumb but elsewhere.

What CSS can I use to force Opera to paint everywhere in the scrollbar? I know that you can set the color of the thumb, up, down, background, etc, for the scrollbar under Explorer, but what about Opera?

Thanks!

Peter