Hi Dave,
That only exists in the Artisteer skins and is one of the files exported by Artisteer, it isn't something generated by mojoPortal and it isn't used in mojoPortal for anthing other than comparing the mojoPortal skin to what the exported Artisteer html looks like.
Note that I don't recommend using Artisteer skins as a starting point for further customization. If you want to modify any of the Artisteer designs it is best to do that in Artisteer and then re-export. We make the .artx files used by Artisteer available also on our codeplex download page in the ArtisteerFiles.zip, you can open those files in Artisteer and then customize the design in Artisteer and re-export the CSS.
For those who want to design from scratch using CSS its better to start with one of the non-Artisteer skins in our extra skins download.
Hope that helps,
Joe