How can I find out if the present theme version would work
Install the Upgrade Status module and then visit /admin/reports/upgrade-status and scan the Adaptive Theme and your sub-theme and get a report on all problems regarding Drupal 9 compatibility.


  This module scans the code of the contributed and custom projects you
  have installed, and reports on any deprecated code that must be
  replaced before the next major version. Available project updates are
  also suggested to keep your site up to date as project will resolve
  deprecation errors over time.
  
  Due to third party PHP library dependencies, the module needs to be
  installed with Composer.