The next advanced feature that you could build into Moodle is creating a separate course theme.
Sometimes, educators might want a completely different look and feel for their courses, or require a special course theme for accessibility issues. They might, for instance, be teaching students specifically with visual disabilities, in which case a specific course theme would need to be built. Another scenario where a course theme could be implemented would be for testing. It is perfectly normal for some theme designers to build their theme inside a course to test the features before implementing it globally across their whole Moodle site.
If you can remember back as far as Chapter 2, Moodle Themes, you will know that Moodle themes have a priority, and are applied hierarchically in this order:
Site > User > Session > Category > Course > Page.
To create a course theme, you would just need to create a dummy course, maybe with the Moodle features demo course material added to it, and set Allow course themes in the Theme settings page in the administration block. Then just go to the settings for that course and choose the theme that you are working on.
You could call this theme something like "Do not use" so that educators do not choose it while you are working on it.