NGINX: Allow 403 custom error page

I have a question about custom error pages, especially the 403 one.

I have this configuration for my vhost :

server {
...
error_page 403 /403.html
...
location / {
    ...
    allow xx.xx.xx.xx;
    deny all;
    }
location /403.html {
    root /var/www/html/error_pages;
    allow all;
    }
}

The file 403.html itself is well served. But the page calls some css and images in /var/www/html/error_pages/css/file.css and /var/www/html/error_pages/img/file.png

And since I have a whitelist on “location /” all files called by 403.html get a 403 error while I put “allow all;” for “location /403.html”.

What am i doing wrong?

(nginx 1.16, debian 8)