In Chrome DEV tools check the Content Type for the css file.
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If it is not text/css you may have a incomplete Nginx configuration.
Check the default Nginx conf file usually in /etc/nginx has the Mimetypes listed as below in the http section:
http{
include /etc/nginx/mime.types;
sendfile on;
}
Nginx Docs
https://www.nginx.com/resources/wiki/start/topics/examples/full/