I have a weird problem. I'm using a custom subtheme of Bartik (but the same issue happens if I use the unmodified Bartik theme).
I've added some CSS background images to the #page-wrapper and #header ids. When I'm logged in as UID=0 then the admin overlay looks as expected with white bg. However if I'm logged in as another user, then the overlay now has #page-wrapper and #header IDs in it, causing the website background images to show up in the overlay! In other words it seems to be using the page.tpl.php file to show the content of the overlay. I can't think why it would/should be doing this. Any ideas?