Could you not skin it with Skinner?
Or use one of the complete UI skinning art packs like:
http://www.wowinterface.com/download...Pack-Blue.html
or
http://www.wowinterface.com/download...48-AI-Art.html
or
http://www.wowinterface.com/download...89-Aurora.html
which will change your bags too (along with the other UI components).