How to set Bartik back by default on Drupal 8?

Question

Drupal

I set Zircon by default - and got a white site with one line "Go to main content": now this line is visible in the admin panel and in the external site.
I removed themes to fix it - didn't work.
I edited the system.theme line in the {config} database and downloaded the BLOB file there and replaced the word zircon with bartik in it and uploaded it again - it did not help.
I returned the themes folder, but Drupal 8 finally broke down. I cleared the cache.
Now I have a site with html headers and one line of content: go to main content. and he does not even set the encoding .. what to do?

Answer 1

the developers here helped me https://www.drupal.org/node/2615944
it turns out that I need to run update.php.