Okay first you can easily host many different Ghost instances on cloudron and use them as dev/test/stage/prod. Cloudron (Premium plan) has Gitlab as app, so you can create your own CI pipeline to deploy your themes. Since the rest of your questions do not really have to do with cloudron I would suggest you this link: https://jamesfacts.com/building-a-continuous-integration-pipeline-for-your-ghost-theme/