Issue #3293469: [Plan] Automated A11y tests in Nightwatch

core/modules/system/tests/modules/nightwatch_theme_install_utility/src/Controller/ThemeInstallController.php

+<?php
+
+namespace Drupal\nightwatch_theme_install_utility\Controller;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Controller\ControllerBase;
+use Drupal\Core\Extension\ThemeInstallerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides an easier way for Nightwatch tests to install themes.
+ */
+class ThemeInstallController extends ControllerBase {
+
+  /**
+   * The theme installer service.
+   *
+   * @var \Drupal\Core\Extension\ThemeInstallerInterface
+   */
+  protected $themeInstaller;
+
+  /**
+   * Constructs a new ThemeInstallController.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The config factory.
+   * @param \Drupal\Core\Extension\ThemeInstallerInterface $theme_installer
+   *   The theme installer.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory, ThemeInstallerInterface $theme_installer) {
+    $this->configFactory = $config_factory;
+    $this->themeInstaller = $theme_installer;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('config.factory'),
+      $container->get('theme_installer')
+    );
+  }
+
+  /**
+   * Install a theme as default.
+   *
+   * @param string $theme
+   *   The theme to install as the default theme.
+   *
+   * @return array
+   *   A render array confirming installation.
+   */
+  public function installDefault(string $theme) {
+    return $this->installTheme($theme, 'default');
+  }
+
+  /**
+   * Install a theme as the admin theme.
+   * @param string $theme
+   *   The theme to install as the admin theme.
+   *
+   * @return array
+   *   A render array confirming installation.
+   */
+  public function installAdmin($theme) {
+    return $this->installTheme($theme, 'admin');
+  }
+
+  /**
+   * Installs a theme.
+   *
+   * @param string $theme
+   *   The theme to install.
+   * @param string $default_or_admin
+   *   Which type of theme to install, can be `default` or `admin`.
+   *
+   * @return array
+   *   A render array confirming installation.
+   */
+  private function installTheme($theme, $default_or_admin): array {
+    assert(in_array($default_or_admin, ['default', 'admin']), 'The $default_or_admin parameter must be `default` or `admin`');
+    $config = $this->configFactory->getEditable('system.theme');
+    $this->themeInstaller->install([$theme]);
+    $config->set($default_or_admin, $theme)->save();
+    return [
+      '#type' => 'container',
+      '#attributes' => ['id' => 'theme-installed'],
+      '#markup' => "Installed $theme as $default_or_admin theme",
+    ];
+  }
+
+}