Currently most (if not all) Flamingo themes rely on external fonts from fonts.googleapis.com. Here is one example.
The problem with this solution is that in some cases the wiki user cannot access fonts.googleapis.com. One enterprise-like example is when the user machine is connected behind a proxy server that can deny access to some external URLs or ask for an authentication in order to access external domains.
Therefore, one idea to fix this problem would be to directly download the fonts themselves and bundle them with Flamingo.