When I sell an item, I use Dreamweaver (a web design tool) to create a nice looking page. This can incorporate backgrounds, javascript etc to give a nice layout. Then I copy the entire page ( <BODY> to </BODY> tags ) into the eBay selling page box. Voila! Instant Karma! That's how it's done...
I always try to leave the background to the whole page as normal, just my 'bit' in the middle is customised this way. I also do clever things with the image that eBay loads for you so that it appears in the middle of the listing - all easy stuff for a web designer with no social life!