ok, I guess I figured out the issue with the error screen, in that I was trying to add the webdynpro abap application to the Theme Designer in portal. So apparently there is a theme designer in portal, and a theme designer on the abap side, and as far as I can tell they are not related (themes do not reside in one location)?
This is confusing to me, and all the documentation I can find online glosses over or ignores this.