I think I was a little too fast in accepting the answer in the thread “How to remove the theme completely”.
If I follow this recipe, then switch to another theme, and remove the Sydney theme, install Sydney again and activate it, much of the setup from the old Sydney site is still there. Parts deleted back then are still deleted etc.
What I’m looking for is a way to remove the theme completely, with all its settings, install Sydney again, and then be at the same stage I was in, just after installing Sydney the first time.
Is this possible?