Look and feel formatting in theme plugin is ignored

I have created a theme plugin which successfully loads, and the home page override works as expected. No errors or warnings are generated during the plugin registration process.

However, none of the look and feel formatting appears in the Confluence space that uses the theme. I have set all of the required properties and validated my atlassian-connect.json file using the Atlassian Connect Validator, but none of the formatting takes.

Has anyone else hit this and found a workaround?