Mobi format does not do CSS, per se. I believe that tools have to convert the css and html into tags that mobi format supports (like old-fashioned HTML before CSS). I don't see anything like a background color in the supported tags list for Mobipocket (http://www.mobipocket.com/dev/articl...TagRef_OEB.htm
). You might try making the image itself having a wide black border, and just use it that way.