I placed my installation behind a nginx reverse proxy, everything works fine but i cant get my theming, if i go back to the installation without the proxy the theming is fine, but as soon as I open the site behind the proxy I lost all the theming.
I set the value of overwritewebroot to the path relative from nginx to nextcloud, which is in my case 'overwritewebroot' => '/nextcloud',
It has been quite a while since you asked the question, so probably you have found another way to deal with it, but maybe someone else will find this answer helpful.