reverted:
--- b/core/modules/system/src/Tests/Menu/MenuRouterTest.php
+++ a/core/modules/system/src/Tests/Menu/MenuRouterTest.php
@@ -306,6 +306,7 @@
 
     $this->drupalGet('menu-test/theme-callback/use-stark-theme');
     $this->assertText('Active theme: stark. Actual theme: stark.', 'The theme negotiation system uses an optional theme once it has been installed.');
+    $this->assertRaw('stark/css/layout.css', "The optional theme's CSS appears on the page.");
 
     $theme_handler->uninstall(array('stark'));
   }
diff -u b/core/themes/stark/stark.info.yml b/core/themes/stark/stark.info.yml
--- b/core/themes/stark/stark.info.yml
+++ b/core/themes/stark/stark.info.yml
@@ -3,4 +3,6 @@
-description: 'An intentionally plain theme with no styling to demonstrate default Drupal’s HTML and CSS. Learn how to build a custom theme from Stark in the <a href="http://drupal.org/theme-guide">Theming Guide</a>.'
+description: 'An intentionally plain theme with almost no styling to demonstrate default Drupal’s HTML and CSS. Learn how to build a custom theme from Stark in the <a href="http://drupal.org/theme-guide">Theming Guide</a>.'
 package: Core
 version: VERSION
 core: 8.x
+libraries:
+  - stark/global-styling
reverted:
--- /dev/null
+++ a/core/themes/stark/stark.libraries.yml
@@ -0,0 +1,5 @@
+global-styling:
+  version: VERSION
+  css:
+    base:
+      css/layout.css: {}