Recovering From A Mangled Theme

I was upgrading a theme on one of my test sites (Drupal 8) like this:

drush up mayo

Something went ... wrong. I'm new to drush, so I can't say much more than that. It failed and my site started acting like I didn't have any themes installed.

I first tried to back out of it by copying the original theme files back out of drush backups, but my site failed to recognize that the files were there. Even after multiple "drush cr". I then backed out of it by setting the default theme to Bartik via drush, and then installed the theme like this:

drush pm-uninstall mayo
... and manually deleting the files from the folder.

I then re-downloaded and re-installed it:

drush pm-download mayo
drush pm-enable mayo
drush cr

Now, what I have is a bare-bones set of Mayo settings under the setting tab (none of the settings that are unique to Mayo) and the theme itself is *not* listed under the list tab. At this point, I can either make it partially exist, like this, or completely go away.

Luckily, this is a test site, but what's the "right" way to recover from something like this?

Thanks!