Well, I wouldn't say it's so much because we aren't using a Portal. It's more because the version of UBB (the software that runs JavaRanch forums) doesn't have that feature.
There is other forum software, like phpBB, that allows the user to use any of the installed themes if they want, and if the admin has allowed that in the configuration of the site.
That being said, I doubt we would allow that even if it were an option. We are all kind of partial to what we have going and we want everyone to suffer with us.