The 'testing' install profile's setting of a default theme (Classy) is now deprecated

Problem/Motivation

---- Drupal\Tests\imce_modal\Functional\LoadTest ----


Status    Group      Filename          Line Function                            
--------------------------------------------------------------------------------
Fail      Other      phpunit-9.xml        0 Drupal\Tests\imce_modal\Functional\
    PHPunit Test failed to complete; Error: PHPUnit 7.5.20 by Sebastian
    Bergmann and contributors.
    
    Testing Drupal\Tests\imce_modal\Functional\LoadTest
    .                                                                   1 / 1
    (100%)
    
    Time: 24.99 seconds, Memory: 8.00 MB
    
    OK (1 test, 8 assertions)
    
    HTML output was generated
    http://localhost/sites/simpletest/browser_output/Drupal_Tests_imce_modal_Functional_LoadTest-11-10970218.html
    http://localhost/sites/simpletest/browser_output/Drupal_Tests_imce_modal_Functional_LoadTest-12-10970218.html
    http://localhost/sites/simpletest/browser_output/Drupal_Tests_imce_modal_Functional_LoadTest-13-10970218.html
    http://localhost/sites/simpletest/browser_output/Drupal_Tests_imce_modal_Functional_LoadTest-14-10970218.html
    http://localhost/sites/simpletest/browser_output/Drupal_Tests_imce_modal_Functional_LoadTest-15-10970218.html
    
    Remaining deprecation notices (1)
    
      1x: Drupal\Tests\BrowserTestBase::$defaultTheme is required in
    drupal:9.0.0 when using an install profile that does not set a default
    theme. See https://www.drupal.org/node/3083055, which includes
    recommendations on which theme to use.
        1x in LoadTest::testLoad from Drupal\Tests\imce_modal\Functional

Proposed resolution

Override property $defaultTheme setting her with value 'stable';