diff -u b/switch_page_theme.info.yml b/switch_page_theme.info.yml
--- b/switch_page_theme.info.yml
+++ b/switch_page_theme.info.yml
@@ -3,6 +3,6 @@
 package: User Interface
 type: module
-core: 8.x
 core_version_requirement: ^8 || ^9
+core: 8.x
 
 configure: switch_page_theme.settings
only in patch2:
unchanged:
--- a/src/Form/SwitchPageThemeSettingForm.php
+++ b/src/Form/SwitchPageThemeSettingForm.php
@@ -3,6 +3,7 @@
 namespace Drupal\switch_page_theme\Form;
 
 use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Extension\ThemeHandlerInterface;
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Language\LanguageManagerInterface;
@@ -21,6 +22,13 @@ class SwitchPageThemeSettingForm extends ConfigFormBase {
    */
   protected $moduleHandler;
 
+  /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
   /**
    * The Domain loader.
    *
@@ -38,8 +46,9 @@ class SwitchPageThemeSettingForm extends ConfigFormBase {
   /**
    * {@inheritdoc}
    */
-  public function __construct(ModuleHandlerInterface $module_handler, DomainLoader $domain_loader = NULL, LanguageManagerInterface $language_manager = NULL) {
+  public function __construct(ModuleHandlerInterface $module_handler, ThemeHandlerInterface $theme_handler, DomainLoader $domain_loader = NULL, LanguageManagerInterface $language_manager = NULL) {
     $this->moduleHandler = $module_handler;
+    $this->themeHandler = $theme_handler;
     if ($domain_loader) {
       $this->domainLoader = $domain_loader;
     }
@@ -62,6 +71,7 @@ class SwitchPageThemeSettingForm extends ConfigFormBase {
     }
     return new static(
       $container->get('module_handler'),
+      $container->get('theme_handler'),
       $domainServices,
       $languageServices
     );
@@ -143,7 +153,7 @@ class SwitchPageThemeSettingForm extends ConfigFormBase {
     ];
 
     // Available themes.
-    $themes = system_list('theme');
+    $themes = $this->themeHandler->listInfo();
     $themeNames[''] = '--Select--';
     foreach ($themes as $key => $value) {
       $themeNames[$key] = $value->info['name'];
only in patch2:
unchanged:
--- a/src/Theme/PageThemeNegotiator.php
+++ b/src/Theme/PageThemeNegotiator.php
@@ -6,7 +6,7 @@ use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\Path\CurrentPathStack;
-use Drupal\Core\Path\AliasManagerInterface;
+use Drupal\path_alias\AliasManagerInterface;
 use Drupal\Core\Path\PathMatcherInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Session\AccountProxyInterface;
@@ -36,7 +36,7 @@ class PageThemeNegotiator implements ThemeNegotiatorInterface {
   /**
    * Protected pathAlias variable.
    *
-   * @var Drupal\Core\Path\AliasManagerInterface
+   * @var Drupal\path_alias\AliasManagerInterface
    */
   protected $pathAlias;
 
only in patch2:
unchanged:
--- a/switch_page_theme.services.yml
+++ b/switch_page_theme.services.yml
@@ -1,6 +1,6 @@
 services:
   theme.negotiator.switch_page_theme:
     class: Drupal\switch_page_theme\Theme\PageThemeNegotiator
-    arguments: ['@config.factory', '@path.current','@path.alias_manager', '@path.matcher', '@current_user', '@module_handler', '@request_stack', '@?domain.negotiator', '@?language_manager']
+    arguments: ['@config.factory', '@path.current', '@path_alias.manager', '@path.matcher', '@current_user', '@module_handler', '@request_stack', '@?domain.negotiator', '@?language_manager']
     tags:
       - { name: theme_negotiator, priority: 1 }