diff --git a/core/modules/system/src/Tests/Menu/MenuRouterTest.php b/core/modules/system/src/Tests/Menu/MenuRouterTest.php
index 1b6bb2e..200d8f4 100644
--- a/core/modules/system/src/Tests/Menu/MenuRouterTest.php
+++ b/core/modules/system/src/Tests/Menu/MenuRouterTest.php
@@ -398,7 +398,7 @@ public function testThemeIntegration() {
   protected function doTestThemeCallbackAdministrative() {
     $this->drupalGet('menu-test/theme-callback/use-admin-theme');
     $this->assertText('Active theme: seven. Actual theme: seven.', 'The administrative theme can be correctly set in a theme negotiation.');
-    $this->assertRaw('seven/style.css', "The administrative theme's CSS appears on the page.");
+    $this->assertRaw('seven/css/style.css', "The administrative theme's CSS appears on the page.");
   }
 
   /**
@@ -417,7 +417,7 @@ protected function doTestThemeCallbackMaintenanceMode() {
     $this->drupalLogin($admin_user);
     $this->drupalGet('menu-test/theme-callback/use-admin-theme');
     $this->assertText('Active theme: seven. Actual theme: seven.', 'The theme negotiation system is correctly triggered for an administrator when the site is in maintenance mode.');
-    $this->assertRaw('seven/style.css', "The administrative theme's CSS appears on the page.");
+    $this->assertRaw('seven/css/style.css', "The administrative theme's CSS appears on the page.");
 
     $this->container->get('state')->set('system.maintenance_mode', FALSE);
   }
diff --git a/core/modules/taxonomy/src/Tests/ThemeTest.php b/core/modules/taxonomy/src/Tests/ThemeTest.php
index ee8cc38..727a969 100644
--- a/core/modules/taxonomy/src/Tests/ThemeTest.php
+++ b/core/modules/taxonomy/src/Tests/ThemeTest.php
@@ -45,7 +45,7 @@ function testTaxonomyTermThemes() {
     // should use the administrative theme.
     $vocabulary = $this->createVocabulary();
     $this->drupalGet('admin/structure/taxonomy/manage/' . $vocabulary->id() . '/add');
-    $this->assertRaw('seven/style.css', t("The administrative theme's CSS appears on the page for adding a taxonomy term."));
+    $this->assertRaw('seven/css/style.css', t("The administrative theme's CSS appears on the page for adding a taxonomy term."));
 
     // Viewing a taxonomy term should use the default theme.
     $term = $this->createTerm($vocabulary);
@@ -54,6 +54,6 @@ function testTaxonomyTermThemes() {
 
     // Editing a taxonomy term should use the same theme as adding one.
     $this->drupalGet('taxonomy/term/' . $term->id() . '/edit');
-    $this->assertRaw('seven/style.css', t("The administrative theme's CSS appears on the page for editing a taxonomy term."));
+    $this->assertRaw('seven/css/style.css', t("The administrative theme's CSS appears on the page for editing a taxonomy term."));
   }
 }
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
index 077150b..082995a 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -171,8 +171,8 @@ public function testRebuildThemeData() {
     // Ensure that the css paths are set with the proper prefix.
     $this->assertEquals(array(
       'screen' => array(
-        'seven.base.css' => DRUPAL_ROOT . '/core/themes/seven/seven.base.css',
-        'style.css' => DRUPAL_ROOT . '/core/themes/seven/style.css',
+        'css/seven.base.css' => DRUPAL_ROOT . '/core/themes/seven/css/seven.base.css',
+        'css/style.css' => DRUPAL_ROOT . '/core/themes/seven/css/style.css',
         'css/components/buttons.css' => DRUPAL_ROOT . '/core/themes/seven/css/components/buttons.css',
         'css/components/buttons.theme.css' => DRUPAL_ROOT . '/core/themes/seven/css/components/buttons.theme.css',
         'css/components/dropbutton.component.css' => DRUPAL_ROOT . '/core/themes/seven/css/components/dropbutton.component.css',
diff --git a/core/themes/seven/install-page.css b/core/themes/seven/css/install-page.css
similarity index 100%
rename from core/themes/seven/install-page.css
rename to core/themes/seven/css/install-page.css
diff --git a/core/themes/seven/jquery.ui.theme.css b/core/themes/seven/css/jquery.ui.theme.css
similarity index 100%
rename from core/themes/seven/jquery.ui.theme.css
rename to core/themes/seven/css/jquery.ui.theme.css
diff --git a/core/themes/seven/maintenance-page.css b/core/themes/seven/css/maintenance-page.css
similarity index 100%
rename from core/themes/seven/maintenance-page.css
rename to core/themes/seven/css/maintenance-page.css
diff --git a/core/themes/seven/quickedit.css b/core/themes/seven/css/quickedit.css
similarity index 100%
rename from core/themes/seven/quickedit.css
rename to core/themes/seven/css/quickedit.css
diff --git a/core/themes/seven/seven.base.css b/core/themes/seven/css/seven.base.css
similarity index 100%
rename from core/themes/seven/seven.base.css
rename to core/themes/seven/css/seven.base.css
diff --git a/core/themes/seven/style.css b/core/themes/seven/css/style.css
similarity index 100%
rename from core/themes/seven/style.css
rename to core/themes/seven/css/style.css
diff --git a/core/themes/seven/vertical-tabs.css b/core/themes/seven/css/vertical-tabs.css
similarity index 100%
rename from core/themes/seven/vertical-tabs.css
rename to core/themes/seven/css/vertical-tabs.css
diff --git a/core/themes/seven/seven.info.yml b/core/themes/seven/seven.info.yml
index b0c4217..8913f52 100644
--- a/core/themes/seven/seven.info.yml
+++ b/core/themes/seven/seven.info.yml
@@ -8,20 +8,20 @@ version: VERSION
 core: 8.x
 stylesheets:
   screen:
-    - seven.base.css
-    - style.css
+    - css/seven.base.css
+    - css/style.css
     - css/components/buttons.css
     - css/components/buttons.theme.css
     - css/components/dropbutton.component.css
     - css/components/tour.theme.css
 stylesheets-override:
-  - vertical-tabs.css
-  - jquery.ui.theme.css
+  - css/vertical-tabs.css
+  - css/jquery.ui.theme.css
   - css/dialog.theme.css
 stylesheets-remove:
   - jquery.ui.dialog.css
 quickedit_stylesheets:
-  - quickedit.css
+  - css/quickedit.css
 regions:
   content: Content
   help: Help
diff --git a/core/themes/seven/seven.libraries.yml b/core/themes/seven/seven.libraries.yml
index fb22ac1..cc4efee 100644
--- a/core/themes/seven/seven.libraries.yml
+++ b/core/themes/seven/seven.libraries.yml
@@ -4,7 +4,7 @@ maintenance-page:
     js/mobile.install.js: {}
   css:
     theme:
-      maintenance-page.css: {}
+      css/maintenance-page.css: {}
   dependencies:
     - system/maintenance
 
@@ -14,7 +14,7 @@ install-page:
     js/mobile.install.js: {}
   css:
     theme:
-      install-page.css: {}
+      css/install-page.css: {}
   dependencies:
     - system/maintenance