diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 7580d0b..7bbd4ff 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -325,6 +325,8 @@ function drupal_find_theme_templates($cache, $extension, $path) {
  * sources:
  * - the saved values from the global theme settings form
  * - the saved values from the theme's settings form
+ * To only retrieve the default global theme setting, an empty string should be
+ * given for $theme.
  *
  * @param $setting_name
  *   The name of the setting to be retrieved.
@@ -346,13 +348,13 @@ function theme_get_setting($setting_name, $theme = NULL) {
   if (empty($cache[$theme])) {
     // Create a theme settings object.
     $cache[$theme] = new ThemeSettings($theme);
+    // Get the global settings from configuration.
+    $cache[$theme]->setData(\Drupal::config('system.theme.global')->get());
+
     $themes = \Drupal::service('theme_handler')->listInfo();
     if (isset($themes[$theme])) {
       $theme_object = $themes[$theme];
 
-      // Get the global settings from configuration.
-      $cache[$theme]->setData(\Drupal::config('system.theme.global')->get());
-
       // Retrieve configured theme-specific settings, if any.
       try {
         if ($theme_settings = \Drupal::config($theme . '.settings')->get()) {
diff --git a/core/lib/Drupal/Core/Config/ConfigInstaller.php b/core/lib/Drupal/Core/Config/ConfigInstaller.php
index e99397e..17d3a07 100644
--- a/core/lib/Drupal/Core/Config/ConfigInstaller.php
+++ b/core/lib/Drupal/Core/Config/ConfigInstaller.php
@@ -90,8 +90,9 @@ public function __construct(ConfigFactoryInterface $config_factory, StorageInter
    */
   public function installDefaultConfig($type, $name) {
     $extension_path = drupal_get_path($type, $name);
-    // If the extension provides configuration schema clear the definitions.
-    if (is_dir($extension_path . '/' . InstallStorage::CONFIG_SCHEMA_DIRECTORY)) {
+    // If the extension provides configuration schema or is a theme clear the
+    // definitions.
+    if (is_dir($extension_path . '/' . InstallStorage::CONFIG_SCHEMA_DIRECTORY) || $type == 'theme') {
       // Refresh the schema cache if installing default configuration and the
       // extension has a configuration schema directory.
       $this->typedConfig->clearCachedDefinitions();
diff --git a/core/modules/config/src/Tests/DefaultConfigTest.php b/core/modules/config/src/Tests/DefaultConfigTest.php
index 4880561..99ff07a 100644
--- a/core/modules/config/src/Tests/DefaultConfigTest.php
+++ b/core/modules/config/src/Tests/DefaultConfigTest.php
@@ -10,7 +10,9 @@
 use Drupal\config_test\TestInstallStorage;
 use Drupal\Core\Config\InstallStorage;
 use Drupal\Core\Config\TypedConfigManager;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\simpletest\KernelTestBase;
+use Symfony\Component\DependencyInjection\Reference;
 
 /**
  * Tests that default configuration provided by all modules matches schema.
@@ -24,23 +26,46 @@ class DefaultConfigTest extends KernelTestBase {
   /**
    * Modules to enable.
    *
+   * Enable the system module so that system_config_schema_info_alter() fires.
+   *
+   * @var array
+   */
+  public static $modules = array('system', 'config_test');
+
+  /**
+   * Themes which provide default configuration and need enabling.
+   *
+   * If a theme provides default configuration but does not have a schema
+   * because it can rely on schemas added by system_config_schema_info_alter()
+   * then this test needs to enable it.
+   *
    * @var array
    */
-  public static $modules = array('config_test');
+  protected $themes = ['seven'];
+
+  protected function setUp() {
+    parent::setUp();
+    \Drupal::service('theme_handler')->install($this->themes);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function containerBuild(ContainerBuilder $container) {
+    parent::containerBuild($container);
+    $container->register('DefaultConfigTest.schema_storage')
+      ->setClass('\Drupal\config_test\TestInstallStorage')
+      ->addArgument(InstallStorage::CONFIG_SCHEMA_DIRECTORY);
+
+    $definition = $container->getDefinition('config.typed');
+    $definition->replaceArgument(1, new Reference('DefaultConfigTest.schema_storage'));
+  }
 
   /**
    * Tests default configuration data type.
    */
   public function testDefaultConfig() {
-    // Create a typed config manager with access to configuration schema in
-    // every module, profile and theme.
-    $typed_config = new TypedConfigManager(
-      \Drupal::service('config.storage'),
-      new TestInstallStorage(InstallStorage::CONFIG_SCHEMA_DIRECTORY),
-      \Drupal::service('cache.discovery'),
-      \Drupal::service('module_handler')
-    );
-
+    $typed_config = \Drupal::service('config.typed');
     // Create a configuration storage with access to default configuration in
     // every module, profile and theme.
     $default_config_storage = new TestInstallStorage();
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 9334a88..e084a5a 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -1344,3 +1344,20 @@ function system_path_insert() {
 function system_path_delete($path) {
   \Drupal::service('path.alias_manager')->cacheClear();
 }
+
+/**
+ * Implements hook_config_schema_info_alter().
+ */
+function system_config_schema_info_alter(&$definitions) {
+  // Provide a default schema for all theme settings files if the theme does
+  // provide its own schema already..
+  foreach (\Drupal::service('theme_handler')->listInfo(TRUE) as $name => $theme) {
+    $config_name = $name . '.settings';
+    if (!isset($definitions[$config_name])) {
+      $definitions[$config_name] = array(
+        'type' => 'theme_settings',
+        'label' => $name . ' settings',
+      );
+    }
+  }
+}
diff --git a/core/themes/bartik/config/schema/bartik.schema.yml b/core/themes/bartik/config/schema/bartik.schema.yml
deleted file mode 100644
index ef355e3..0000000
--- a/core/themes/bartik/config/schema/bartik.schema.yml
+++ /dev/null
@@ -1,5 +0,0 @@
-# Schema for the configuration files of the Bartik theme.
-
-bartik.settings:
-  type: theme_settings
-  label: 'Bartik settings'
diff --git a/core/themes/seven/config/schema/seven.schema.yml b/core/themes/seven/config/schema/seven.schema.yml
deleted file mode 100644
index 6d4c86e..0000000
--- a/core/themes/seven/config/schema/seven.schema.yml
+++ /dev/null
@@ -1,9 +0,0 @@
-# Schema for the configuration files of the Seven theme.
-
-seven.settings:
-  type: theme_settings
-  label: 'Seven settings'
-
-seven.breakpoints:
-  type: theme_breakpoints_default
-  label: 'Seven breakpoints'