TestInstallerThemeHooks.php

<?php

declare (strict_types=1);
namespace Drupal\test_installer_theme\Hook;

use Drupal\Core\Hook\Attribute\Hook;

/**
 * Form alter hooks for installer tests.
 */
class TestInstallerThemeHooks {
  
  /**
   * Implements hook_form_FORM_ID_alter().
   */
  public function formInstallSelectLanguageFormAlter(array &$form) : void {
    $form['function_name']['#markup'] = 'Added by custom installer theme.';
  }

}