Soo, there appeared to be two errors. One with the nginx config which I solved in the past ( a / typo behind the proxy_pass ) for anyone having this same issue make sure you double check the nginx rules for a typo.
@jongarrison you have another example on nodebb-plugin-custom-header. But in this case is managed by a plugin too. I don't know if it is possible to applu different theme (tpl). You could also managed it in the same tpl with a little bit of logic in it based in certain configuration server side.