Figured that out now too!
For FusionBB users, under "Forum Settings" you need to type the name of your style sheet into:
Default theme setting (UBBThreads users only)
Even though it says UBBThreads users only, it works with FusionBB customers as well. In my case, I'm using the default style sheet so I typed "Professional" in that space (the name of the stylesheet I use) and boom!
Great to be back up and running again!