diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index 4eae7ae..f52b4ba 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -216,7 +216,7 @@ function testListThemes() {
     // Check if ThemeHandlerInterface::listInfo() returns disabled themes.
     // Check for base theme and subtheme lists.
     $base_theme_list = array('test_basetheme' => 'Theme test base theme');
-    $sub_theme_list = array('test_subsubtheme' => 'Theme test subsubtheme', 'test_subtheme' => 'Theme test subtheme');
+    $sub_theme_list = array('test_subsubtheme' => 'Theme test subsubtheme', 'test_subtheme' => 'Theme test subtheme', 'test_theme_alter' => 'Theme test alter');
 
     $this->assertIdentical($themes['test_basetheme']->sub_themes, $sub_theme_list, 'Base theme\'s object includes list of subthemes.');
     $this->assertIdentical($themes['test_subtheme']->base_themes, $base_theme_list, 'Subtheme\'s object includes list of base themes.');
diff --git a/core/modules/system/src/Tests/Theme/ThemeTestCustomSettings.php b/core/modules/system/src/Tests/Theme/ThemeTestCustomSettings.php
new file mode 100644
index 0000000..e3a0409
--- /dev/null
+++ b/core/modules/system/src/Tests/Theme/ThemeTestCustomSettings.php
@@ -0,0 +1,73 @@
+<?php
+
+namespace Drupal\system\Tests\Theme;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests custom theme settings.
+ *
+ * @group Theme
+ */
+class ThemeTestCustomSettings extends WebTestBase {
+
+  /**
+   * Modules to install.
+   *
+   * @var array
+   */
+  public static $modules = ['file'];
+
+  /**
+   * A user with administrative permissions.
+   *
+   * @var \Drupal\user\UserInterface
+   */
+  protected $bigUser;
+
+  protected function setUp() {
+    parent::setUp();
+
+    // Create user.
+    $this->bigUser = $this->drupalCreateUser(['administer themes']);
+    // Install theme.
+    \Drupal::service('theme_handler')->install(['test_theme_alter']);
+  }
+
+  /**
+   * Tests extend theme settings with custom fields and custom submit handlers.
+   */
+  function testCustomThemeSettings() {
+    $theme = 'test_theme_alter';
+
+    $this->config('system.theme')
+      ->set('default', $theme)
+      ->save();
+
+    $settings_path = 'admin/appearance/settings/' . $theme;
+
+    $this->drupalLogin($this->bigUser);
+
+    $this->drupalGet($settings_path);
+    $this->assertResponse(200);
+    // Altered fields.
+    $this->assertFieldByName('toggle_favicon', FALSE);
+    // Custom fields.
+    $this->assertFieldByName('custom_flag', FALSE);
+    $this->assertFieldByName('custom_text', '');
+    $this->assertFieldByName('files[custom_logo]', '');
+
+    // Edit options.
+    $image_file = current($this->drupalGetTestFiles('image'));
+    $edit = [
+      'custom_flag' => TRUE,
+      'custom_text' => 'Tests text',
+      'files[custom_logo]' => drupal_realpath($image_file->uri),
+    ];
+    $this->drupalPostForm($settings_path, $edit, t('Save configuration'));
+    $this->assertFieldByName('custom_flag', $edit['custom_flag']);
+    $this->assertFieldByName('custom_text', $edit['custom_text']);
+    $this->assertFieldByName('files[custom_logo]', $edit['files[custom_logo]']);
+  }
+
+}
diff --git a/core/modules/system/tests/themes/test_theme_alter/config/schema/test_theme_alter.schema.yml b/core/modules/system/tests/themes/test_theme_alter/config/schema/test_theme_alter.schema.yml
new file mode 100644
index 0000000..6bb97f5
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_alter/config/schema/test_theme_alter.schema.yml
@@ -0,0 +1,17 @@
+# Schema for the configuration files of the Test theme alter.
+
+test_theme_alter.settings:
+  type: theme_settings
+  label: 'Test theme alter settings'
+  mapping:
+    custom_flag:
+      type: boolean
+      label: 'Custom flag'
+    custom_text:
+      type: string
+      label: 'Custom text'
+    custom_logo:
+      type: sequence
+      label: 'Custom logo'
+      sequence:
+        - type: uri
diff --git a/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.info.yml b/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.info.yml
new file mode 100644
index 0000000..d718e05
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.info.yml
@@ -0,0 +1,6 @@
+name: 'Theme test alter'
+type: theme
+description: 'Test theme alter that extends theme settings options'
+version: VERSION
+core: 8.x
+base theme: test_basetheme
diff --git a/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.theme b/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.theme
new file mode 100644
index 0000000..04628c8
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.theme
@@ -0,0 +1,12 @@
+<?php
+
+/**
+ * @file
+ * Add hooks for tests to use.
+ */
+
+/**
+ * Implements hook_preprocess_HOOK() for theme_test_template_test templates.
+ */
+function test_subsubtheme_preprocess_theme_test_template_test(&$variables) {
+}
diff --git a/core/modules/system/tests/themes/test_theme_alter/theme-settings.php b/core/modules/system/tests/themes/test_theme_alter/theme-settings.php
new file mode 100644
index 0000000..58d34fd
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_alter/theme-settings.php
@@ -0,0 +1,60 @@
+<?php
+
+/**
+ * Implements hook_form_system_theme_settings_alter().
+ */
+function test_theme_alter_form_system_theme_settings_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
+  // Alter default field
+  $form['toggle_favicon']['#default_value'] = FALSE;
+
+  // Add custom fields
+  $form['extra_settings'] = array(
+    '#type' => 'vertical_tabs',
+  );
+  $form['extra'] = array(
+    '#type' => 'details',
+    '#title' => t('Extra'),
+    '#group' => 'extra_settings',
+  );
+  $form['extra']['custom_flag'] = array(
+    '#type' => 'checkbox',
+    '#title' => t('Custom flag.'),
+    '#default_value' => theme_get_setting('custom_flag'),
+  );
+  $form['extra']['custom_text'] = array(
+    '#type' => 'textfield',
+    '#title' => t('Custom textfield.'),
+    '#default_value' => theme_get_setting('custom_text'),
+  );
+  $default_file_dir = 'public://template/logos';
+  $logo_folder = file_prepare_directory($default_file_dir, FILE_CREATE_DIRECTORY);
+  $form['extra']['custom_logo'] = array(
+    '#type' => 'managed_file',
+    '#title' => t('Secondary logo.'),
+    '#default_value' => theme_get_setting('custom_logo'),
+    '#progress_indicator' => 'bar',
+    '#progress_message'   => t('Please wait...'),
+    '#upload_location' => $logo_folder,
+    '#upload_validators'  => array(
+      'file_validate_extensions' => array('gif png jpg jpeg'),
+      'file_validate_image_resolution' => array('30x50', '88x100'),
+    ),
+  );
+  // Add a new submission handler.
+  $form['#submit'][] = 'test_theme_alter_form_system_theme_settings_submit';
+}
+
+/**
+ * Test theme form settings submission handler.
+ */
+function test_theme_alter_form_system_theme_settings_submit(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
+  if ($form_state->getValue('custom_logo')) {
+    $element = $form_state->getValue('custom_logo');
+    if (!empty($element[0])){
+      // Make submited files permanent.
+      $file = \Drupal\file\Entity\File::load($fid);
+      $file->setPermanent();
+      $file->save();
+    }
+  }
+}