Missing schema definition for bootstrap_barrio_button_type

When running phpunit tests and setting the default theme to my sub-theme of bootstrap_barrio, I get the following error message:

1) Drupal\Tests\my_module\Functional\LoadTest::testLoad
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for bootstrap_barrio.settings with the following errors: bootstrap_barrio.settings:bootstrap_barrio_button_type missing schema

/var/www/html/drupal/web/core/lib/Drupal/Core/Config/Development/ConfigSchemaChecker.php:94
/var/www/html/drupal/web/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php:142
/var/www/html/drupal/web/core/lib/Drupal/Core/Config/Config.php:229
/var/www/html/drupal/web/core/lib/Drupal/Core/Config/ConfigInstaller.php:399
/var/www/html/drupal/web/core/lib/Drupal/Core/Config/ConfigInstaller.php:152
/var/www/html/drupal/web/core/lib/Drupal/Core/ProxyClass/Config/ConfigInstaller.php:75
/var/www/html/drupal/web/core/lib/Drupal/Core/Extension/ThemeInstaller.php:235
/var/www/html/drupal/web/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php:440
/var/www/html/drupal/web/core/tests/Drupal/Tests/BrowserTestBase.php:544
/var/www/html/drupal/web/core/tests/Drupal/Tests/BrowserTestBase.php:364
/var/www/html/drupal/web/modules/custom/dhr_migrate_ui/tests/src/Functional/LoadTest.php:38
/var/www/html/drupal/vendor/phpunit/phpunit/src/Framework/TestResult.php:728

I think we need a schema definition because it is a setting used elsewhere:

https://git.drupalcode.org/project/bootstrap_barrio/-/blob/5.5.x/theme-s...

I would be interested to know if other people have encountered this issue?