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.';
  }
  
  /**
   * Implements hook_form_FORM_ID_alter().
   *
   * The site configuration form is the last installer step. The profile has
   * installed its own themes by then, so this marker shows that the install
   * theme still has its hook implementations registered.
   */
  public function formInstallConfigureFormAlter(array &$form) : void {
    $form['installer_theme_marker'] = [
      '#markup' => 'Added by custom installer theme on the last step.',
      '#weight' => -100,
    ];
  }

}