It would be great if there was also a changelog for developers. This would highlight CSS changes and describe which parts of the CSS code have been changed or added.
This is probably less interesting for users and more important for CSS developers. Especially with character limits, it makes sense to be able to delete "dead code" or patch elements more specifically without having to re-examine every single page. This would significantly improve a developer's workflow.
Perhaps you could also show in the header which pages are currently affected by the custom CSS or which ones are not yet working. Another idea is to associate important elements with an image.
