I wanted to simply customise the header of my blog (to add a verification link) and I noticed I could not edit the default theme (there is no Edit Theme button like your documentation says).
So I installed a new one and there is no Edit button there as well.
Last but not least, there doesn’t seem to be an obvious way to uninstall the new theme and go back to the default one or to simple select a different theme.
It was changed after the “soft” (unannounced) launch. But @mantonsaid he can re-enable the features he removed for anyone who subscribed before it was all ironed out. If you email help@micro.blog, I expect he’ll get you sorted.