Sorry but I don't offer any support for modifying Artisteer designs outside of Artisteer. Using Artisteer you could get the .artx file from our ArtisteerFiles.zip on a previous release of mojoPortal at codeplex. Then you could change the background color from within Artisteer and export a new mojoPortal skin using our free skin exporter for Artisteer.

Note that this question was moved to the design forum from the questions about site administration forum where it was originally posted.