diff -u b/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
--- b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -70,8 +70,9 @@
     // This is desired, because missing or malformed theme configuration should
     // not leave the application in a broken state. By falling back to default
     // output, the user is able to reconfigure the theme through the UI.
-    // In addition, kernel tests are expected to operate with no theme and only
-    // the original theme output of modules by default.
+    // Lastly, tests are expected to operate with no theme by default, so as to
+    // only assert the original theme output of modules (unless a test manually
+    // enables a specific theme).
     if (empty($themes) || !$theme_name || !isset($themes[$theme_name])) {
       $theme_name = 'core';
       // /core/core.info.yml does not actually exist, but is required because
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Extension/DefaultConfigTest.php
@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Extension\DefaultConfigTest.
+ */
+
+namespace Drupal\Tests\Core\Extension;
+
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Yaml\Yaml;
+
+/**
+ * Tests default configuration of the Extension system.
+ *
+ * @group Extension
+ */
+class DefaultConfigTest extends UnitTestCase {
+
+  /**
+   * Tests that core.extension.yml is empty by default.
+   *
+   * The default configuration MUST NOT specify any extensions, because every
+   * extension has to be installed in a regular way.
+   *
+   * Otherwise, the regular runtime application would operate with extensions
+   * that were never installed. The default configuration of such extensions
+   * would not exist. Installation hooks would never be executed.
+   */
+  public function testConfigIsEmpty() {
+    $config = Yaml::parse(file_get_contents(DRUPAL_ROOT . '/core/config/install/core.extension.yml'));
+    $expected = array(
+      'module' => array(),
+      'theme' => array(),
+      'disabled' => array(
+        'theme' => array(),
+      ),
+    );
+    $this->assertEquals($expected, $config);
+  }
+
+}