diff --git a/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php b/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php
index 750e967..ba55764 100644
--- a/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php
@@ -8,7 +8,7 @@
 namespace Drupal\Core\Installer\Form;
 
 use Drupal\Core\Extension\ModuleInstallerInterface;
-use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Locale\CountryManagerInterface;
 use Drupal\Core\State\StateInterface;
@@ -18,7 +18,7 @@
 /**
  * Provides the site configuration form.
  */
-class SiteConfigureForm extends FormBase {
+class SiteConfigureForm extends ConfigFormBase {
 
   /**
    * The user storage.
diff --git a/core/modules/language/src/Form/NegotiationConfigureForm.php b/core/modules/language/src/Form/NegotiationConfigureForm.php
index c7495e9..ca20801 100644
--- a/core/modules/language/src/Form/NegotiationConfigureForm.php
+++ b/core/modules/language/src/Form/NegotiationConfigureForm.php
@@ -14,7 +14,7 @@
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Extension\ThemeHandlerInterface;
-use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Url;
 use Drupal\language\ConfigurableLanguageManagerInterface;
@@ -25,7 +25,7 @@
 /**
  * Configure the selected language negotiation method for this site.
  */
-class NegotiationConfigureForm extends FormBase {
+class NegotiationConfigureForm extends ConfigFormBase {
 
   /**
    * Stores the configuration object for language.types.
@@ -86,7 +86,8 @@ class NegotiationConfigureForm extends FormBase {
    *   The block storage, or NULL if not available.
    */
   public function __construct(ConfigFactoryInterface $config_factory, ConfigurableLanguageManagerInterface $language_manager, LanguageNegotiatorInterface $negotiator, BlockManagerInterface $block_manager, ThemeHandlerInterface $theme_handler, EntityStorageInterface $block_storage = NULL) {
-    $this->languageTypes = $config_factory->get('language.types');
+    parent::__construct($config_factory);
+    $this->languageTypes = $this->config('language.types');
     $this->languageManager = $language_manager;
     $this->negotiator = $negotiator;
     $this->blockManager = $block_manager;
diff --git a/core/modules/system/src/Form/ThemeAdminForm.php b/core/modules/system/src/Form/ThemeAdminForm.php
index 2cee8c5..93cace8 100644
--- a/core/modules/system/src/Form/ThemeAdminForm.php
+++ b/core/modules/system/src/Form/ThemeAdminForm.php
@@ -6,13 +6,13 @@
 
 namespace Drupal\system\Form;
 
-use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
 
 /**
  * Form to select the administration theme.
  */
-class ThemeAdminForm extends FormBase {
+class ThemeAdminForm extends ConfigFormBase {
 
   /**
    * {@inheritdoc}
@@ -50,7 +50,7 @@ public function buildForm(array $form, FormStateInterface $form_state, array $th
    * {@inheritdoc}
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
-    drupal_set_message($this->t('The configuration options have been saved.'));
+    parent::submitForm($form, $form_state);
     $this->config('system.theme')->set('admin', $form_state->getValue('admin_theme'))->save();
   }