This error occured when upgrading to drupal 9. The solution is to update your mytheme.info file with the following key:
base_theme: false
Also make sure it has the following:
core_version_requirement: ^8 || ^9
This error occured when upgrading to drupal 9. The solution is to update your mytheme.info file with the following key:
base_theme: false
Also make sure it has the following:
core_version_requirement: ^8 || ^9