Typing "actiontabs" into the theme box does what I'd expect it do, but typing "simple" doesn't. Does it have a different name maybe?

It's not really a theme, but instead the lack of any theme. Try making the text box empty.
Comment by joeyh.name Fri Apr 11 17:55:29 2014
I tried that. I also tried uploading empty style.css and local.css files.
Comment by Thomas Sat Apr 12 16:16:08 2014
Ah! I think I just needed to rebuild the wiki. Now it's simple.
Comment by Thomas Sat Apr 12 16:19:36 2014