Hi Steve,
I’ve replied to you via email, but I’m also adding it here in case this information would be useful to other users who are facing similar issue.
Graphene and Graphene Plus does not have any hardcoded URLs to CSS files. Typically issues like these occur when there are existing entries in the database that are using HTTP instead of HTTPS. When migrating an existing site from HTTP to HTTPS, it is almost always required to do a search and replace on the database entries as well. It is not sufficient to simply enable SSL on the server.
If your host is unable to assist you with that, I would suggest for you to try installing the Really Simple SSL plugin. This would be the simplest way to move an existing WordPress site to HTTPS.