[#3297262]: Update method of getting list of installed themes.

Compatible back to 8.7.0.

modules/form_mode_theme_switcher/form_mode_manager_theme_switcher.links.menu.yml

 # Admin UI - Theme switcher
 form_mode_manager.theme_switcher_settings:
   title: 'Form Mode theme switcher'
-  description: 'xxx'
+  description: 'Allows configuration of the theme used by the entity form modes.'
   weight: -10
   parent: form_mode_manager.admin_settings
   route_name: form_mode_manager.theme_switcher_settings

modules/form_mode_theme_switcher/src/Form/FormModeThemeSwitcherForm.php

@@ -6,6 +6,7 @@ use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Entity\EntityDisplayRepositoryInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Extension\ThemeExtensionList;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\form_mode_manager\Form\FormModeManagerFormBase;
 use Drupal\form_mode_manager\FormModeManagerInterface;
@@ -16,6 +17,15 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
  */
 class FormModeThemeSwitcherForm extends FormModeManagerFormBase {
 
+  /**
+   * Provides a list of available themes.
+   *
+   * @var \Drupal\Core\Extension\ThemeExtensionList
+   *
+   * @see https://www.drupal.org/project/drupal/issues/2940481
+   */
+  protected $themeList;
+
   /**
    * Constructs a CropWidgetForm object.
    *
@@ -29,10 +39,26 @@ class FormModeThemeSwitcherForm extends FormModeManagerFormBase {
    *   The cache tags invalidator.
    * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
    *   The entity type manager service.
+   * @param \Drupal\Core\Extension\ThemeExtensionList $theme_extension_list
+   *   The theme extension list service.
    */
-  public function __construct(ConfigFactoryInterface $config_factory, EntityDisplayRepositoryInterface $entity_display_repository, FormModeManagerInterface $form_mode_manager, CacheTagsInvalidatorInterface $cache_tags_invalidator, EntityTypeManagerInterface $entity_type_manager) {
-    parent::__construct($config_factory, $entity_display_repository, $form_mode_manager, $cache_tags_invalidator, $entity_type_manager);
+  public function __construct(
+    ConfigFactoryInterface $config_factory,
+    EntityDisplayRepositoryInterface $entity_display_repository,
+    FormModeManagerInterface $form_mode_manager,
+    CacheTagsInvalidatorInterface $cache_tags_invalidator,
+    EntityTypeManagerInterface $entity_type_manager,
+    ThemeExtensionList $theme_extension_list
+  ) {
+    parent::__construct(
+      $config_factory,
+      $entity_display_repository,
+      $form_mode_manager,
+      $cache_tags_invalidator,
+      $entity_type_manager
+    );
 
+    $this->themeList = $theme_extension_list;
     $this->ignoreExcluded = FALSE;
     $this->ignoreActiveDisplay = FALSE;
   }
@@ -46,7 +72,8 @@ class FormModeThemeSwitcherForm extends FormModeManagerFormBase {
       $container->get('entity_display.repository'),
       $container->get('form_mode.manager'),
       $container->get('cache_tags.invalidator'),
-      $container->get('entity_type.manager')
+      $container->get('entity_type.manager'),
+      $container->get('extension.list.theme')
     );
   }
 
@@ -81,9 +108,11 @@ class FormModeThemeSwitcherForm extends FormModeManagerFormBase {
    * {@inheritdoc}
    */
   public function getActiveThemeOptions() {
+    $installed_themes = $this->themeList->getAllInstalledInfo();
+
     $options = [];
-    foreach ($this->configFactory->get('system.theme.data') as $theme_name => $theme_extension) {
-      $options[$theme_name] = $theme_extension->info['name'];
+    foreach ($installed_themes as $theme_name => $theme_extension) {
+      $options[$theme_name] = $theme_extension['name'];
     }
 
     return $options;