Hi,
You are trying to use the release files in VS, if you used the source code .sln this error would not happen.
But you can probably resolve it by deleting the files WebForm1.aspx, WebForm1.aspx.cs, and WebForm1.aspx.designer.cs from the /Data/skins/dcarter-bluedesert folder. Those files don't need to be there, I guess I didn't notice those files when this skin was contributed, I'm deleting them from my copy now.
I generally recommend developers to use the source code download because there are many good code examples in mojoPortal todo almost anyting you might need to do and looking at that code can help you with your custom development.
Best,
Joe