This happens because one of XenForo's default style properties, which by default stores a number and which the wiki CSS uses a XenForo math function on, has had its content cleared out by the Flat Awesome style. This is fixed in the next release by ensuring that the style property is a number before passing it to the math function.
Note that CSS errors can only be seen in XenForo's debug mode. Since the demo is a test site, we have debug mode on a lot of the time.