Made some changes I cannot revert

Not easily. We’ll merge it into Discourse within the next few weeks, and then you’ll be able to update your site. In the meantime, we have the solution @Falco shared

Sorry, I probably confused things by mentioning components. You can do the same thing with regular themes: create a fresh one and use the preview link.

If you haven’t already read it, this is a good place to get started: