Hey Luca,
Regretfully, we do not have X 6.1.0 available for download anymore. The stable release of X is 6.4.6. You can download it from your Apex Dashboard.
I logged in to your cPanel however, there’s no control in there that could potentially fix the 403 Forbidden issue. This issue can only be resolved by your web host because they have access to all server controls.
Please specifically ask your host why wp-content/plugins/cornerstone/assets/dist/css/app/app.css is forbidden in your site. That will answer why the rest of the CSS is forbidden.

Can you also try asking your host if they can switch your web server to Apache at least temporarily just to test if the type of server is causing the issue.
Please try downgrading to the stable release of X to see if that fixes the issue for now.
Thanks.