Added 'theme -> ...' line to config.php, internal server error on all sreens

Nextcloud version: Not sure how to check, as I can’t log in. Should be latest, as I used official docker-compose examples
Operating system and version: Ubuntu 18.04
Apache or nginx version: 1.14.1
PHP version: 7.3.5

The issue you are facing:

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

  1. Follow the README here -, add the contents to a themes/nextcloud-breeze-dark theme folder
  2. Add the 'theme' => 'nextcloud-breeze-dark', line to config.php, as per the above ^ README
  3. Refresh the nextcloud UI - internal server error on all pages (including not being able to log in, etc.)
  4. Remove the theme line added to config.php above, as well as the themes/nextcloud-breeze-dark folder above
  5. Refresh the web app on any page - internal server error remains

The output of your Nextcloud log in Admin > Logging:

N/A - can't access any page due to internal server error

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'htaccess.RewriteBase' => '/',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'apps_paths' => 
  array (
    0 => 
    array (
      'path' => '/var/www/html/apps',
      'url' => '/apps',
      'writable' => false,
    1 => 
    array (
      'path' => '/var/www/html/custom_apps',
      'url' => '/custom_apps',
      'writable' => true,
  'instanceid' => '<redacted>',
  'passwordsalt' => '<redacted>',
  'secret' => '<redacted>',
  'trusted_domains' => 
  array (
    0 => '<redacted>',
  'datadirectory' => '/var/www/html/data',
  'dbtype' => 'pgsql',
  'version' => '',
  'overwrite.cli.url' => '<redacted>',
  'dbname' => 'nextcloud',
  'dbhost' => 'db',
  'dbport' => '',
  'dbtableprefix' => '',
  'dbuser' => '<redacted>',
  'dbpassword' => '<redacted>',
  'installed' => true,                                                                                               

The output of your Apache/nginx/system log in /var/log/____:
N/A. the /var/log/nginx/access.log and /var/log/nginx/error.log files in my nginx container are completely empty.
For good measure I checked /var/log/apache2/access.log and /var/log/apache2/error.log are totally empty as well
(the latter is expected as I’m using nginx and not apache).

Note that before I made the config.php theme changes, the instance was working perfectly fine over HTTPS, and I had
already been up and running with several apps including Tasks, Contacts, etc. and had synced successfully with both mobile
and other desktop clients.