Themes

1. See if theme has a 'db' folder, if so, remove any tables defined in a contained xml file.
   
2. Delete the theme folder.
   
3. Delete rows pertaining to the theme in the 'config_plugins' table.
   
4. Clear the value of 'allversionshash' in the 'config' table.
   
5. Change the value of 'theme' in the 'config' table to a stable one, such as 'clean'.
6. 'Purge all caches' - use CLI version if cannot use UI.
   
7. See if anything needs to be done when you navigate to the site admin 'Notifications'.
   

or you could....

1. Change the value of 'theme' in the 'config' table to a stable one.
   
2. Run CLI version of 'Purge all caches'.
   

and see if that gets you out of the problem.  If so, then try and remove broken theme via UI or use steps one to seven above.