Find another rule

FunctionalTestDefaultThemePropertyRector

Adds $defaultTheme property to Functional and FunctionalJavascript tests which do not have them.

 class SomeClassTest {
+  protected $defaultTheme = 'stark'
 }

Configure your rector.php:

<?php

use DrupalRector\Drupal8\Rector\Deprecation\FunctionalTestDefaultThemePropertyRector;
use Rector\Config\RectorConfig;

return RectorConfig::configure()
    ->withRules([
        FunctionalTestDefaultThemePropertyRector::class,
    ]);