Ok, I put Helius into my PHP-Nuke themes folder, and now the site works (forums included). The only thing I cannot access is the adminstration for the forums, where I get the critical error. I wasn't thinking to just switch to the Helius PHP-Nuke themes, so I moved the forums folder from the Helius folder to /modules/forums/templates, installed the theme in the phpBB configuration menu, then switched to it.

Now I need a way, sans the admin/config phpBB menu, to switch back to subSilver as the phpBB theme (I have access to the actual server, so I don't need to use FTP).