diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index f7651d2..466e959 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -331,7 +331,13 @@ public function buildForm(array $form, FormStateInterface $form_state, $theme =
         foreach ($filenames as $filename) {
           if (file_exists($filename)) {
             require_once $filename;
-            $form_state->addBuildInfo('files', [$filename]);
+
+            // The file must be require for cached form too.
+            $files = $form_state->getBuildInfo()['files'];
+            if (!in_array($filename, $files)) {
+              $files[] = $filename;
+            }
+            $form_state->addBuildInfo('files', $files);
           }
         }
 
diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index 6df1fec..64f330e 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -216,7 +216,7 @@ public function testListThemes() {
     // Check if ThemeHandlerInterface::listInfo() returns disabled themes.
     // Check for base theme and subtheme lists.
     $base_theme_list = ['test_basetheme' => 'Theme test base theme'];
-    $sub_theme_list = ['test_subsubtheme' => 'Theme test subsubtheme', 'test_subtheme' => 'Theme test subtheme', 'test_theme_alter' => 'Theme test alter'];
+    $sub_theme_list = ['test_subsubtheme' => 'Theme test subsubtheme', 'test_subtheme' => 'Theme test subtheme'];
 
     $this->assertIdentical($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/tests/src/Functional/Theme/ThemeTestCustomSettings.php b/core/modules/system/tests/src/Functional/Theme/ThemeTestCustomSettings.php
deleted file mode 100644
index 824e908..0000000
--- a/core/modules/system/tests/src/Functional/Theme/ThemeTestCustomSettings.php
+++ /dev/null
@@ -1,121 +0,0 @@
-<?php
-
-namespace Drupal\Tests\system\Functional\Theme;
-
-use Drupal\file\Entity\File;
-use Drupal\Tests\BrowserTestBase;
-use Drupal\Tests\TestFileCreationTrait;
-
-/**
- * Tests custom theme settings.
- *
- * @group Theme
- */
-class ThemeTestCustomSettings extends BrowserTestBase {
-
-  use TestFileCreationTrait {
-    getTestFiles as drupalGetTestFiles;
-  }
-
-  /**
-   * 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 = $this->drupalCreateUser(['administer themes']);
-    // Install theme.
-    \Drupal::service('theme_handler')->install(['test_theme', 'test_theme_alter']);
-  }
-
-  /**
-   * Retrieves a sample file of the specified type.
-   *
-   * @return \Drupal\file\FileInterface
-   */
-  protected function getTestFile($type_name, $size = NULL) {
-    // Get a file to upload.
-    $file = current($this->drupalGetTestFiles($type_name, $size));
-
-    // Add a filesize property to files as would be read by
-    // \Drupal\file\Entity\File::load().
-    $file->filesize = filesize($file->uri);
-
-    return File::create((array) $file);
-  }
-
-  /**
-   * Retrieves the fid of the last inserted file.
-   */
-  protected function getLastFileId() {
-    return (int) db_query('SELECT MAX(fid) FROM {file_managed}')->fetchField();
-  }
-
-  /**
-   * Tests extended theme settings with custom fields and submit handlers.
-   */
-  public function testCustomThemeSettings() {
-    $themes = ['test_theme', 'test_theme_alter'];
-
-    foreach ($themes as $theme) {
-      $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);
-
-      // Add a new managed file.
-      $image_file = $this->getTestFile('image');
-      $image_file_path = drupal_realpath($image_file->getFileUri());
-      $edit = [
-        'files[custom_logo]' => $image_file_path,
-      ];
-      // Submit the managed_file form.
-      $this->drupalPostForm(NULL, $edit, t('Upload'));
-      // Assert the new file is uploaded as temporary. This file should not be
-      // saved as permanent if settings are not submited.
-      $fid1 = $this->getLastFileId();
-      $file1 = File::load($fid1);
-      $this->assertFalse($file1->isPermanent());
-
-      // Edit more options, attach a new file and submit settings form.
-      $edit = [
-        'toggle_favicon' => FALSE,
-        'custom_flag' => TRUE,
-        'custom_text' => 'Tests text',
-        'files[custom_logo]' => $image_file_path,
-      ];
-      $this->drupalPostForm($settings_path, $edit, t('Save configuration'));
-
-      $fid2 = $this->getLastFileId();
-      $this->assertFieldByName('toggle_favicon', $edit['toggle_favicon']);
-      $this->assertFieldByName('custom_flag', $edit['custom_flag']);
-      $this->assertFieldByName('custom_text', $edit['custom_text']);
-
-      // Assert the uploaded file is saved as permanent.
-      $file2 = File::load($fid2);
-      $this->assertTrue($file2->isPermanent());
-      // Assert the first uploaded file is still temporally.
-      $this->assertFalse($file1->isPermanent());
-    }
-
-  }
-
-}
diff --git a/core/modules/system/tests/src/FunctionalJavascript/ThemeFormSettingsTest.php b/core/modules/system/tests/src/FunctionalJavascript/ThemeFormSettingsTest.php
new file mode 100644
index 0000000..8e4729f
--- /dev/null
+++ b/core/modules/system/tests/src/FunctionalJavascript/ThemeFormSettingsTest.php
@@ -0,0 +1,78 @@
+<?php
+
+namespace Drupal\Tests\system\FunctionalJavascript;
+
+use Drupal\file\Entity\File;
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+use Drupal\Tests\TestFileCreationTrait;
+
+/**
+ * Tests that theme form settings works correctly.
+ *
+ * @group system
+ */
+class ThemeFormSettingsTest extends JavascriptTestBase {
+
+  use TestFileCreationTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['file'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $admin = $this->drupalCreateUser(['administer themes']);
+    $this->drupalLogin($admin);
+  }
+
+  /**
+   * Tests that submission handler works correctly.
+   *
+   * @dataProvider providerTestFormSettingsSubmissionHandler
+   */
+  public function testFormSettingsSubmissionHandler($theme) {
+
+    \Drupal::service('theme_handler')->install([$theme]);
+
+    $page = $this->getSession()->getPage();
+    $assert_session = $this->assertSession();
+
+    $this->drupalGet("admin/appearance/settings/$theme");
+
+    // Add a new managed file.
+    $file = current($this->getTestFiles('image'));
+    $image_file_path = \Drupal::service('file_system')->realpath($file->uri);
+    $page->attachFileToField('files[custom_logo]', $image_file_path);
+    $assert_session->waitForButton('custom_logo_remove_button');
+
+    // Assert the new file is uploaded as temporary. This file should not be
+    // saved as permanent if settings are not submited.
+    $image_field = $this->xpath('//input[@name="custom_logo[fids]"]')[0];
+    $file = File::load($image_field->getValue());
+    $this->assertFalse($file->isPermanent());
+
+    $page->pressButton('Save configuration');
+    \Drupal::entityTypeManager()->getStorage('file')->resetCache();
+
+    // Assert the uploaded file is saved as permanent.
+    $image_field = $this->xpath('//input[@name="custom_logo[fids]"]')[0];
+    $file = File::load($image_field->getValue());
+    $this->assertTrue($file->isPermanent());
+  }
+
+  /**
+   * Provides test data for ::testFormSettingsSubmissionHandler().
+   */
+  public function providerTestFormSettingsSubmissionHandler() {
+    return [
+      'test theme.theme' => ['test_theme_theme'],
+      'test theme-settings.php' => ['test_theme_settings'],
+    ];
+  }
+
+}
diff --git a/core/modules/system/tests/themes/test_theme/config/schema/test_theme.schema.yml b/core/modules/system/tests/themes/test_theme/config/schema/test_theme.schema.yml
deleted file mode 100644
index 31af721..0000000
--- a/core/modules/system/tests/themes/test_theme/config/schema/test_theme.schema.yml
+++ /dev/null
@@ -1,18 +0,0 @@
-# Schema for the configuration files of the Test theme.
-
-test_theme.settings:
-  type: theme_settings
-  label: 'Test theme 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: integer
-        label: 'fids'
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index 51ef4b8..aa399b66 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -2,14 +2,6 @@
 
 /**
  * @file
- * Test to ensure theme compatibility with managed files.
- */
-
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\file\Entity\File;
-
-/**
- * @file
  * Theme to help test the Twig engine.
  */
 
@@ -162,51 +154,3 @@ function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__flamin
 function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__meerkat__tarsier__moose(&$variables) {
   $variables['bar'] = 'Moose';
 }
-
-/**
- * Implements hook_form_system_theme_settings_alter().
- */
-function test_theme_form_system_theme_settings_alter(&$form, FormStateInterface $form_state) {
-  // Alter default field.
-  $form['toggle_favicon']['#default_value'] = FALSE;
-
-  // Add custom fields.
-  $form['custom_flag'] = [
-    '#type' => 'checkbox',
-    '#title' => t('Custom flag.'),
-    '#default_value' => theme_get_setting('custom_flag'),
-  ];
-  $form['custom_text'] = [
-    '#type' => 'textfield',
-    '#title' => t('Custom textfield.'),
-    '#default_value' => theme_get_setting('custom_text'),
-  ];
-  $logo_path = 'public://test';
-  $success = file_prepare_directory($logo_path, FILE_CREATE_DIRECTORY);
-  $form['custom_logo'] = [
-    '#type' => 'managed_file',
-    '#name' => 'custom_logo',
-    '#title' => t('Secondary logo.'),
-    '#default_value' => theme_get_setting('custom_logo'),
-    '#progress_indicator' => 'bar',
-    '#progress_message' => t('Please wait...'),
-    '#upload_location' => $logo_path,
-    '#upload_validators' => [
-      'file_validate_extensions' => ['gif png jpg jpeg'],
-    ],
-  ];
-  // Add a new submission handler.
-  $form['#submit'][] = 'test_theme_form_system_theme_settings_submit';
-}
-
-/**
- * Test theme form settings submission handler.
- */
-function test_theme_form_system_theme_settings_submit(&$form, FormStateInterface $form_state) {
-  if ($file_id = $form_state->getValue(['custom_logo', '0'])) {
-    // Make submitted files permanent.
-    $file = File::load($file_id);
-    $file->setPermanent();
-    $file->save();
-  }
-}
diff --git a/core/modules/system/tests/themes/test_theme_alter/config/schema/test_theme_alter.schema.yml b/core/modules/system/tests/themes/test_theme_alter/config/schema/test_theme_alter.schema.yml
deleted file mode 100644
index c8be03b..0000000
--- a/core/modules/system/tests/themes/test_theme_alter/config/schema/test_theme_alter.schema.yml
+++ /dev/null
@@ -1,18 +0,0 @@
-# Schema for the configuration files of the Test theme alter.
-
-test_theme_alter.settings:
-  type: theme_settings
-  label: 'Test theme alter settings'
-  mapping:
-    custom_flag:
-      type: boolean
-      label: 'Custom flag'
-    custom_text:
-      type: string
-      label: 'Custom text'
-    custom_logo:
-      type: sequence
-      label: 'Custom logo'
-      sequence:
-        type: integer
-        label: 'fids'
diff --git a/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.info.yml b/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.info.yml
deleted file mode 100644
index d718e05..0000000
--- a/core/modules/system/tests/themes/test_theme_alter/test_theme_alter.info.yml
+++ /dev/null
@@ -1,6 +0,0 @@
-name: 'Theme test alter'
-type: theme
-description: 'Test theme alter that extends theme settings options'
-version: VERSION
-core: 8.x
-base theme: test_basetheme
diff --git a/core/modules/system/tests/themes/test_theme_alter/theme-settings.php b/core/modules/system/tests/themes/test_theme_alter/theme-settings.php
deleted file mode 100644
index 1ecc1da..0000000
--- a/core/modules/system/tests/themes/test_theme_alter/theme-settings.php
+++ /dev/null
@@ -1,56 +0,0 @@
-<?php
-
-/**
- * @file
- * Theme settings to help test custom theme settings.
- */
-
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\file\Entity\File;
-
-/**
- * Implements hook_form_system_theme_settings_alter().
- */
-function test_theme_alter_form_system_theme_settings_alter(&$form, FormStateInterface $form_state) {
-  // Alter default field.
-  $form['toggle_favicon']['#default_value'] = FALSE;
-
-  // Add custom fields.
-  $form['custom_flag'] = [
-    '#type' => 'checkbox',
-    '#title' => t('Custom flag.'),
-    '#default_value' => theme_get_setting('custom_flag'),
-  ];
-  $form['custom_text'] = [
-    '#type' => 'textfield',
-    '#title' => t('Custom textfield.'),
-    '#default_value' => theme_get_setting('custom_text'),
-  ];
-  $logo_path = 'public://test';
-  $success = file_prepare_directory($logo_path, FILE_CREATE_DIRECTORY);
-  $form['custom_logo'] = [
-    '#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_path,
-    '#upload_validators'  => [
-      'file_validate_extensions' => ['gif png jpg jpeg'],
-    ],
-  ];
-  // 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, FormStateInterface $form_state) {
-  if ($file_id = $form_state->getValue(['custom_logo', '0'])) {
-    // Make submitted files permanent.
-    $file = File::load($file_id);
-    $file->setPermanent();
-    $file->save();
-  }
-}
diff --git a/core/modules/system/tests/themes/test_theme_settings/config/schema/test_theme_settings.schema.yml b/core/modules/system/tests/themes/test_theme_settings/config/schema/test_theme_settings.schema.yml
new file mode 100644
index 0000000..a53e94a
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_settings/config/schema/test_theme_settings.schema.yml
@@ -0,0 +1,12 @@
+# Schema for the configuration files of the Test theme settings.
+
+test_theme_settings.settings:
+  type: theme_settings
+  label: 'Test theme settings'
+  mapping:
+    custom_logo:
+      type: sequence
+      label: 'Custom logo'
+      sequence:
+        type: integer
+        label: 'fids'
diff --git a/core/modules/system/tests/themes/test_theme_settings/test_theme_settings.info.yml b/core/modules/system/tests/themes/test_theme_settings/test_theme_settings.info.yml
new file mode 100644
index 0000000..38dd300
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_settings/test_theme_settings.info.yml
@@ -0,0 +1,6 @@
+name: 'Theme test theme-settings.php'
+type: theme
+description: 'Test theme that extends theme settings options via theme-settings.php file'
+version: VERSION
+core: 8.x
+base theme: false
diff --git a/core/modules/system/tests/themes/test_theme_settings/theme-settings.php b/core/modules/system/tests/themes/test_theme_settings/theme-settings.php
new file mode 100644
index 0000000..e82825e
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_settings/theme-settings.php
@@ -0,0 +1,35 @@
+<?php
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\file\Entity\File;
+
+/**
+ * Implements hook_form_system_theme_settings_alter().
+ */
+function test_theme_settings_form_system_theme_settings_alter(&$form, FormStateInterface $form_state) {
+  $form['custom_logo'] = [
+    '#type' => 'managed_file',
+    '#title' => t('Secondary logo.'),
+    '#default_value' => theme_get_setting('custom_logo'),
+    '#progress_indicator' => 'bar',
+    '#progress_message'   => t('Please wait...'),
+    '#upload_location' => 'public://test',
+    '#upload_validators'  => [
+      'file_validate_extensions' => ['gif png jpg jpeg'],
+    ],
+  ];
+
+  // Add a new submission handler.
+  $form['#submit'][] = 'test_theme_settings_form_system_theme_settings_submit';
+}
+
+/**
+ * Test theme form settings submission handler.
+ */
+function test_theme_settings_form_system_theme_settings_submit(&$form, FormStateInterface $form_state) {
+  if ($file_id = $form_state->getValue(['custom_logo', '0'])) {
+    $file = File::load($file_id);
+    $file->setPermanent();
+    $file->save();
+  }
+}
diff --git a/core/modules/system/tests/themes/test_theme_theme/config/schema/test_theme_theme.schema.yml b/core/modules/system/tests/themes/test_theme_theme/config/schema/test_theme_theme.schema.yml
new file mode 100644
index 0000000..fcd044e
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_theme/config/schema/test_theme_theme.schema.yml
@@ -0,0 +1,12 @@
+# Schema for the configuration files of the Test theme theme.
+
+test_theme_theme.settings:
+  type: theme_settings
+  label: 'Test theme settings'
+  mapping:
+    custom_logo:
+      type: sequence
+      label: 'Custom logo'
+      sequence:
+        type: integer
+        label: 'fids'
diff --git a/core/modules/system/tests/themes/test_theme_theme/test_theme_theme.info.yml b/core/modules/system/tests/themes/test_theme_theme/test_theme_theme.info.yml
new file mode 100644
index 0000000..c67a4be
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_theme/test_theme_theme.info.yml
@@ -0,0 +1,6 @@
+name: 'Theme test theme.theme file'
+type: theme
+description: 'Test theme that extends theme settings options via theme.theme file'
+version: VERSION
+core: 8.x
+base theme: false
diff --git a/core/modules/system/tests/themes/test_theme_theme/test_theme_theme.theme b/core/modules/system/tests/themes/test_theme_theme/test_theme_theme.theme
new file mode 100644
index 0000000..00f214a
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_theme/test_theme_theme.theme
@@ -0,0 +1,36 @@
+<?php
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\file\Entity\File;
+
+/**
+ * Implements hook_form_system_theme_settings_alter().
+ */
+function test_theme_theme_form_system_theme_settings_alter(&$form, FormStateInterface $form_state) {
+
+  $form['custom_logo'] = [
+    '#type' => 'managed_file',
+    '#title' => t('Secondary logo.'),
+    '#default_value' => theme_get_setting('custom_logo'),
+    '#progress_indicator' => 'bar',
+    '#progress_message'   => t('Please wait...'),
+    '#upload_location' => 'public://test',
+    '#upload_validators'  => [
+      'file_validate_extensions' => ['gif png jpg jpeg'],
+    ],
+  ];
+
+  // Add a new submission handler.
+  $form['#submit'][] = 'test_theme_theme_form_system_theme_settings_submit';
+}
+
+/**
+ * Test theme form settings submission handler.
+ */
+function test_theme_theme_form_system_theme_settings_submit(&$form, FormStateInterface $form_state) {
+  if ($file_id = $form_state->getValue(['custom_logo', '0'])) {
+    $file = File::load($file_id);
+    $file->setPermanent();
+    $file->save();
+  }
+}