diff --git a/core/core.libraries.yml b/core/core.libraries.yml
index 3d5fcefba0fadd9d1b748a5a9231e4c3f63e79bb..3e933c5c9807e9323c32846b7d818e18b5413562 100644
--- a/core/core.libraries.yml
+++ b/core/core.libraries.yml
@@ -161,9 +161,6 @@ drupal.dialog:
     misc/dialog/dialog.js: {}
     misc/dialog/dialog.position.js: {}
     misc/dialog/dialog.jquery-ui.js: {}
-  css:
-    theme:
-      misc/dialog.theme.css: {}
   dependencies:
     - core/jquery
     - core/drupal
@@ -199,8 +196,6 @@ drupal.dropbutton:
   css:
     component:
       misc/dropbutton/dropbutton.css: {}
-    theme:
-      misc/dropbutton/dropbutton.theme.css: {}
   dependencies:
     - core/jquery
     - core/drupal
diff --git a/core/modules/file/css/file.theme.css b/core/modules/file/css/file.theme.css
deleted file mode 100644
index e17e2b83c5c45cf5d285133ab96ee358f0250fec..0000000000000000000000000000000000000000
--- a/core/modules/file/css/file.theme.css
+++ /dev/null
@@ -1,62 +0,0 @@
-/**
- * @file
- * Default style for file module.
- */
-
-/* File icons.*/
-
-.file {
-  padding-left: 20px; /* LTR */
-  display: inline-block;
-  min-height: 16px;
-  background-repeat: no-repeat;
-  background-position: left center; /* LTR */
-}
-[dir="rtl"] .file {
-  padding-left: inherit;
-  padding-right: 20px;
-  background-position: right center;
-}
-.file--general,
-.file--application-octet-stream {
-  background-image: url(../icons/application-octet-stream.png);
-}
-.file--package-x-generic {
-  background-image: url(../icons/package-x-generic.png);
-}
-.file--x-office-spreadsheet {
-  background-image: url(../icons/x-office-spreadsheet.png);
-}
-.file--x-office-document {
-  background-image: url(../icons/x-office-document.png);
-}
-.file--x-office-presentation {
-  background-image: url(../icons/x-office-presentation.png);
-}
-.file--text-x-script {
-  background-image: url(../icons/text-x-script.png);
-}
-.file--text-html {
-  background-image: url(../icons/text-html.png);
-}
-.file--text-plain {
-  background-image: url(../icons/text-plain.png);
-}
-.file--application-pdf {
-  background-image: url(../icons/application-pdf.png);
-}
-.file--application-x-executable {
-  background-image: url(../icons/application-x-executable.png);
-}
-.file--audio {
-  background-image: url(../icons/audio-x-generic.png);
-}
-.file--video {
-  background-image: url(../icons/video-x-generic.png);
-}
-.file--text {
-  background-image: url(../icons/text-x-generic.png);
-}
-.file--image {
-  background-image: url(../icons/image-x-generic.png);
-}
diff --git a/core/modules/file/file.libraries.yml b/core/modules/file/file.libraries.yml
index c44af55c5e8da3edd96af2a5588c5c426a379145..f72a2eb61569fff7df81ae6329e4869f67d19ce5 100644
--- a/core/modules/file/file.libraries.yml
+++ b/core/modules/file/file.libraries.yml
@@ -12,10 +12,3 @@ drupal.file:
     - core/jquery.once
     - core/drupal
     - core/drupalSettings
-
-drupal.file.formatter.generic:
-  version: VERSION
-  css:
-    theme:
-      css/file.theme.css: {}
-
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php b/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php
index a157d96678f740cd1416ca17793332b09e24fd58..6219cd00ae183d77eb12d09a6c4ef85d6cabce02 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php
@@ -47,11 +47,6 @@ public function viewElements(FieldItemListInterface $items, $langcode) {
         unset($item->_attributes);
       }
     }
-    if (!empty($elements)) {
-      $elements['#attached'] = array(
-        'library' => array('file/drupal.file.formatter.generic'),
-      );
-    }
 
     return $elements;
   }
diff --git a/core/modules/forum/forum.libraries.yml b/core/modules/forum/forum.libraries.yml
deleted file mode 100644
index bf7079edd7fa47c1794481d8713828fa7ace305f..0000000000000000000000000000000000000000
--- a/core/modules/forum/forum.libraries.yml
+++ /dev/null
@@ -1,5 +0,0 @@
-forum.index:
-  version: VERSION
-  css:
-    component:
-      css/forum.theme.css: {}
diff --git a/core/modules/forum/src/Controller/ForumController.php b/core/modules/forum/src/Controller/ForumController.php
index 7f84a025ec3d4ec87ed0d54c3d3f75512b7c8bc6..7e2a8152c66c9ae40db7741f212e7a1ca5d65779 100644
--- a/core/modules/forum/src/Controller/ForumController.php
+++ b/core/modules/forum/src/Controller/ForumController.php
@@ -224,7 +224,6 @@ protected function build($forums, TermInterface $term, $topics = array(), $paren
       '#sortby' => $config->get('topics.order'),
       '#forums_per_page' => $config->get('topics.page_limit'),
     );
-    $build['#attached']['library'][] = 'forum/forum.index';
     if (empty($term->forum_container->value)) {
       $build['#attached']['feed'][] = array('taxonomy/term/' . $term->id() . '/feed', 'RSS - ' . $term->getName());
     }
diff --git a/core/modules/image/image.libraries.yml b/core/modules/image/image.libraries.yml
index fbf9ef4b2294a4b4f52fb9056a0a6cd7e12bbe71..e9061a42e424560f6bfb455cb4fd1ea85b6101af 100644
--- a/core/modules/image/image.libraries.yml
+++ b/core/modules/image/image.libraries.yml
@@ -1,9 +1,3 @@
-form:
-  version: VERSION
-  css:
-    theme:
-      css/image.theme.css: {}
-
 admin:
   version: VERSION
   css:
diff --git a/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php b/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php
index 1449c7f594768292a07618f39fce7f57249f9468..ec83970d07ecb8b5287290b86daa0ca7fbb18207 100644
--- a/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php
+++ b/core/modules/image/src/Plugin/Field/FieldWidget/ImageWidget.php
@@ -160,7 +160,6 @@ public static function process($element, FormStateInterface $form_state, $form)
     $item['fids'] = $element['fids']['#value'];
 
     $element['#theme'] = 'image_widget';
-    $element['#attached']['library'][] = 'image/form';
 
     // Add the image preview.
     if (!empty($element['#files']) && $element['#preview_image_style']) {
diff --git a/core/modules/user/css/user.icons.theme.css b/core/modules/user/css/user.icons.admin.css
similarity index 100%
rename from core/modules/user/css/user.icons.theme.css
rename to core/modules/user/css/user.icons.admin.css
diff --git a/core/modules/user/user.libraries.yml b/core/modules/user/user.libraries.yml
index e11cbfa29e7ffdac2f302472c33bcf7388b19e2f..73e4e15814686fe538a964a711efb69e86de0e6a 100644
--- a/core/modules/user/user.libraries.yml
+++ b/core/modules/user/user.libraries.yml
@@ -5,8 +5,6 @@ drupal.user:
   css:
     component:
       css/user.module.css: {}
-    theme:
-      css/user.theme.css: {}
   dependencies:
     - core/jquery
     - core/drupal
@@ -33,4 +31,4 @@ drupal.user.icons:
   version: VERSION
   css:
     theme:
-      css/user.icons.theme.css: {}
+      css/user.icons.admin.css: {}
diff --git a/core/themes/classy/classy.libraries.yml b/core/themes/classy/classy.libraries.yml
index 67477a8bcd0caaeee70c412b563e53be0a878a4b..d39d2024518b629cb494cca96f7705b49ed6fc70 100644
--- a/core/themes/classy/classy.libraries.yml
+++ b/core/themes/classy/classy.libraries.yml
@@ -31,6 +31,30 @@ book-navigation:
     component:
       css/components/book-navigation.css: {}
 
+dialog:
+  version: VERSION
+  css:
+    component:
+      css/components/dialog.css: { weight: -10 }
+
+dropbutton:
+  version: VERSION
+  css:
+    component:
+      css/components/dropbutton.css: { weight: -10 }
+
+file:
+  version: VERSION
+  css:
+    component:
+      css/components/file.css: { weight: -10 }
+
+forum:
+  version: VERSION
+  css:
+    component:
+      css/components/forum.css: { weight: -10 }
+
 indented:
   version: VERSION
   css:
@@ -60,3 +84,9 @@ search-results:
   css:
     component:
       css/components/search-results.css: {}
+
+user:
+  version: VERSION
+  css:
+    component:
+      css/components/user.css: { weight: -10 }
diff --git a/core/themes/classy/classy.theme b/core/themes/classy/classy.theme
new file mode 100644
index 0000000000000000000000000000000000000000..bc2cede4bd9b1bb375aefb87041e91944f14b19f
--- /dev/null
+++ b/core/themes/classy/classy.theme
@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * @file
+ * Functions to support theming in the Classy theme.
+ */
+
+/**
+ * Implements hook_library_info_alter().
+ */
+function classy_library_info_alter(&$libraries, $extension) {
+  if ($extension == 'user' && isset($libraries['drupal.user'])) {
+    // @todo Replace with https://www.drupal.org/node/2497667 when possible.
+    $library_discovery = \Drupal::service('library.discovery');
+    $library = $library_discovery->getLibraryByName('classy', 'user');
+    foreach ($library['css'] as $css) {
+      $libraries['drupal.user']['css']['component'][base_path() . $css['data']] = [];
+    }
+  }
+  if ($extension == 'core' && isset($libraries['drupal.dropbutton'])) {
+    $libraries['drupal.dropbutton']['dependencies'][] = 'classy/dropbutton';
+  }
+  if ($extension == 'core' && isset($libraries['drupal.dialog'])) {
+    $libraries['drupal.dialog']['dependencies'][] = 'classy/dialog';
+  }
+}
diff --git a/core/misc/dialog.theme.css b/core/themes/classy/css/components/dialog.css
similarity index 99%
rename from core/misc/dialog.theme.css
rename to core/themes/classy/css/components/dialog.css
index 837e49f63d8ce692e97c8cde8bf20aa7c1d87460..d45d34b1cb3d806b3041e37ee1d41b3090f5dacc 100644
--- a/core/misc/dialog.theme.css
+++ b/core/themes/classy/css/components/dialog.css
@@ -1,4 +1,5 @@
 /**
+ * @file
  * Presentational styles for Drupal dialogs.
  */
 
diff --git a/core/misc/dropbutton/dropbutton.theme.css b/core/themes/classy/css/components/dropbutton.css
similarity index 99%
rename from core/misc/dropbutton/dropbutton.theme.css
rename to core/themes/classy/css/components/dropbutton.css
index 65f25685b9b86ffd1210f0a33b0bfb5111644caf..cf8c40a238a17c84e658b85e0fa62f5f2f8fc4b9 100644
--- a/core/misc/dropbutton/dropbutton.theme.css
+++ b/core/themes/classy/css/components/dropbutton.css
@@ -1,4 +1,3 @@
-
 /**
  * @file
  * General styles for dropbuttons.
diff --git a/core/themes/classy/css/components/file.css b/core/themes/classy/css/components/file.css
new file mode 100644
index 0000000000000000000000000000000000000000..5e16b4f768c13ecf6bf3b11f3e2fa537cdd33df8
--- /dev/null
+++ b/core/themes/classy/css/components/file.css
@@ -0,0 +1,62 @@
+/**
+ * @file
+ * Default style for file module.
+ */
+
+/* File icons.*/
+
+.file {
+  padding-left: 20px; /* LTR */
+  display: inline-block;
+  min-height: 16px;
+  background-repeat: no-repeat;
+  background-position: left center; /* LTR */
+}
+[dir="rtl"] .file {
+  padding-left: inherit;
+  padding-right: 20px;
+  background-position: right center;
+}
+.file--general,
+.file--application-octet-stream {
+  background-image: url(../../images/icons/application-octet-stream.png);
+}
+.file--package-x-generic {
+  background-image: url(../../images/icons/package-x-generic.png);
+}
+.file--x-office-spreadsheet {
+  background-image: url(../../images/icons/x-office-spreadsheet.png);
+}
+.file--x-office-document {
+  background-image: url(../../images/icons/x-office-document.png);
+}
+.file--x-office-presentation {
+  background-image: url(../../images/icons/x-office-presentation.png);
+}
+.file--text-x-script {
+  background-image: url(../../images/icons/text-x-script.png);
+}
+.file--text-html {
+  background-image: url(../../images/icons/text-html.png);
+}
+.file--text-plain {
+  background-image: url(../../images/icons/text-plain.png);
+}
+.file--application-pdf {
+  background-image: url(../../images/icons/application-pdf.png);
+}
+.file--application-x-executable {
+  background-image: url(../../images/icons/application-x-executable.png);
+}
+.file--audio {
+  background-image: url(../../images/icons/audio-x-generic.png);
+}
+.file--video {
+  background-image: url(../../images/icons/video-x-generic.png);
+}
+.file--text {
+  background-image: url(../../images/icons/text-x-generic.png);
+}
+.file--image {
+  background-image: url(../../images/icons/image-x-generic.png);
+}
diff --git a/core/modules/forum/css/forum.theme.css b/core/themes/classy/css/components/forum.css
similarity index 92%
rename from core/modules/forum/css/forum.theme.css
rename to core/themes/classy/css/components/forum.css
index 1ce5c4daf32c4c2acb7ae1ec3ba1b97781fd24ae..2f715245205b5e7b98067e508d388f8afa45eb6d 100644
--- a/core/modules/forum/css/forum.theme.css
+++ b/core/themes/classy/css/components/forum.css
@@ -8,7 +8,7 @@
   margin: 0.5em;
 }
 .forum__icon {
-  background-image: url(../icons/forum-icons.png);
+  background-image: url(../../images/icons/forum-icons.png);
   background-repeat: no-repeat;
   float: left; /* LTR */
   height: 24px;
diff --git a/core/modules/image/css/image.theme.css b/core/themes/classy/css/components/image-widget.css
similarity index 100%
rename from core/modules/image/css/image.theme.css
rename to core/themes/classy/css/components/image-widget.css
diff --git a/core/modules/user/css/user.theme.css b/core/themes/classy/css/components/user.css
similarity index 100%
rename from core/modules/user/css/user.theme.css
rename to core/themes/classy/css/components/user.css
diff --git a/core/modules/file/icons/application-octet-stream.png b/core/themes/classy/images/icons/application-octet-stream.png
similarity index 100%
rename from core/modules/file/icons/application-octet-stream.png
rename to core/themes/classy/images/icons/application-octet-stream.png
diff --git a/core/modules/file/icons/application-pdf.png b/core/themes/classy/images/icons/application-pdf.png
similarity index 100%
rename from core/modules/file/icons/application-pdf.png
rename to core/themes/classy/images/icons/application-pdf.png
diff --git a/core/modules/file/icons/application-x-executable.png b/core/themes/classy/images/icons/application-x-executable.png
similarity index 100%
rename from core/modules/file/icons/application-x-executable.png
rename to core/themes/classy/images/icons/application-x-executable.png
diff --git a/core/modules/file/icons/audio-x-generic.png b/core/themes/classy/images/icons/audio-x-generic.png
similarity index 100%
rename from core/modules/file/icons/audio-x-generic.png
rename to core/themes/classy/images/icons/audio-x-generic.png
diff --git a/core/modules/forum/icons/forum-icons.png b/core/themes/classy/images/icons/forum-icons.png
similarity index 100%
rename from core/modules/forum/icons/forum-icons.png
rename to core/themes/classy/images/icons/forum-icons.png
diff --git a/core/modules/file/icons/image-x-generic.png b/core/themes/classy/images/icons/image-x-generic.png
similarity index 100%
rename from core/modules/file/icons/image-x-generic.png
rename to core/themes/classy/images/icons/image-x-generic.png
diff --git a/core/modules/file/icons/package-x-generic.png b/core/themes/classy/images/icons/package-x-generic.png
similarity index 100%
rename from core/modules/file/icons/package-x-generic.png
rename to core/themes/classy/images/icons/package-x-generic.png
diff --git a/core/modules/file/icons/text-html.png b/core/themes/classy/images/icons/text-html.png
similarity index 100%
rename from core/modules/file/icons/text-html.png
rename to core/themes/classy/images/icons/text-html.png
diff --git a/core/modules/file/icons/text-plain.png b/core/themes/classy/images/icons/text-plain.png
similarity index 100%
rename from core/modules/file/icons/text-plain.png
rename to core/themes/classy/images/icons/text-plain.png
diff --git a/core/modules/file/icons/text-x-generic.png b/core/themes/classy/images/icons/text-x-generic.png
similarity index 100%
rename from core/modules/file/icons/text-x-generic.png
rename to core/themes/classy/images/icons/text-x-generic.png
diff --git a/core/modules/file/icons/text-x-script.png b/core/themes/classy/images/icons/text-x-script.png
similarity index 100%
rename from core/modules/file/icons/text-x-script.png
rename to core/themes/classy/images/icons/text-x-script.png
diff --git a/core/modules/file/icons/video-x-generic.png b/core/themes/classy/images/icons/video-x-generic.png
similarity index 100%
rename from core/modules/file/icons/video-x-generic.png
rename to core/themes/classy/images/icons/video-x-generic.png
diff --git a/core/modules/file/icons/x-office-document.png b/core/themes/classy/images/icons/x-office-document.png
similarity index 100%
rename from core/modules/file/icons/x-office-document.png
rename to core/themes/classy/images/icons/x-office-document.png
diff --git a/core/modules/file/icons/x-office-presentation.png b/core/themes/classy/images/icons/x-office-presentation.png
similarity index 100%
rename from core/modules/file/icons/x-office-presentation.png
rename to core/themes/classy/images/icons/x-office-presentation.png
diff --git a/core/modules/file/icons/x-office-spreadsheet.png b/core/themes/classy/images/icons/x-office-spreadsheet.png
similarity index 100%
rename from core/modules/file/icons/x-office-spreadsheet.png
rename to core/themes/classy/images/icons/x-office-spreadsheet.png
diff --git a/core/themes/classy/templates/content-edit/file-managed-file.html.twig b/core/themes/classy/templates/content-edit/file-managed-file.html.twig
index f639237d2a0f4a3f851ba2fb4d82af5315304dbb..e7ae48d3fe2c04a54fed58a71620e587c7dfd2d1 100644
--- a/core/themes/classy/templates/content-edit/file-managed-file.html.twig
+++ b/core/themes/classy/templates/content-edit/file-managed-file.html.twig
@@ -10,6 +10,7 @@
  * @see template_preprocess_file_managed_file()
  */
 #}
+{{ attach_library('classy/file') }}
 {%
   set classes = [
     'js-form-managed-file',
diff --git a/core/themes/classy/templates/content-edit/file-widget.html.twig b/core/themes/classy/templates/content-edit/file-widget.html.twig
index 55de590f57146a2bbc03d9a83245162350e02206..23d03a1ae18eeb5b45648ed7899a9bf8be6172ff 100644
--- a/core/themes/classy/templates/content-edit/file-widget.html.twig
+++ b/core/themes/classy/templates/content-edit/file-widget.html.twig
@@ -10,6 +10,7 @@
  * @see template_preprocess_file_widget()
  */
 #}
+{{ attach_library('classy/file') }}
 <div{{ attributes }}>
   {{ element }}
 </div>
diff --git a/core/themes/classy/templates/content-edit/image-widget.html.twig b/core/themes/classy/templates/content-edit/image-widget.html.twig
index dac3a227b3d3a5eadd45b2538311a1c894fea040..5e8033ffaee2d4f38cc821b8b86769e5e4e7a7e4 100644
--- a/core/themes/classy/templates/content-edit/image-widget.html.twig
+++ b/core/themes/classy/templates/content-edit/image-widget.html.twig
@@ -10,6 +10,7 @@
  * @see template_preprocess_image_widget()
  */
 #}
+{{ attach_library('classy/image-widget') }}
 <div{{ attributes }}>
   {% if data.preview %}
     <div class="image-preview">
diff --git a/core/themes/classy/templates/dataset/forums.html.twig b/core/themes/classy/templates/dataset/forums.html.twig
index eada3a8af7547b489a45fab84d41535e3c138a05..e3f22129a9e83101ed42a9ab8ccfc1e335038cc5 100644
--- a/core/themes/classy/templates/dataset/forums.html.twig
+++ b/core/themes/classy/templates/dataset/forums.html.twig
@@ -14,6 +14,7 @@
  * @see template_preprocess_forums()
  */
 #}
+{{ attach_library('classy/forum') }}
 {% if forums_defined %}
   <div class="forum">
     {{ forums }}
diff --git a/core/themes/classy/templates/field/file-link.html.twig b/core/themes/classy/templates/field/file-link.html.twig
index ad93361ebd73660716449a4f88cac5e56a7767b4..11170c0c0f65644d060bb6e20d5cdd7e5a47955a 100644
--- a/core/themes/classy/templates/field/file-link.html.twig
+++ b/core/themes/classy/templates/field/file-link.html.twig
@@ -11,4 +11,5 @@
  * @see template_preprocess_file_link()
  */
 #}
+{{ attach_library('classy/file') }}
 <span{{ attributes }}>{{ icon }} {{ link }}</span>
diff --git a/core/themes/seven/css/components/dialog.theme.css b/core/themes/seven/css/components/dialog.css
similarity index 100%
rename from core/themes/seven/css/components/dialog.theme.css
rename to core/themes/seven/css/components/dialog.css
diff --git a/core/themes/seven/seven.libraries.yml b/core/themes/seven/seven.libraries.yml
index 764403b6400c4c94567ebe8b0a52c0ce49f772ab..cec9887e27a578b14ca32567fe365d2912906db4 100644
--- a/core/themes/seven/seven.libraries.yml
+++ b/core/themes/seven/seven.libraries.yml
@@ -92,4 +92,4 @@ seven.drupal.dialog:
   version: VERSION
   css:
     theme:
-      css/components/dialog.theme.css: {}
+      css/components/dialog.css: {}
diff --git a/core/themes/seven/seven.theme b/core/themes/seven/seven.theme
index 9727e15d08b831b1273502a232a9610b4e8ed482..7bd56e04443fcc8f764db6f121faf30b5ad70f02 100644
--- a/core/themes/seven/seven.theme
+++ b/core/themes/seven/seven.theme
@@ -186,8 +186,8 @@ function seven_library_info_alter(&$libraries, $extension) {
     unset($libraries['jquery.ui']['css']['theme']['assets/vendor/jquery.ui/themes/base/theme.css']);
     $libraries['jquery.ui']['dependencies'][] = 'seven/seven.jquery.ui';
   }
-  if ($extension == 'core' && isset($libraries['drupal.dialog'])) {
-    unset($libraries['drupal.dialog']['css']['theme']['misc/dialog.theme.css']);
-    $libraries['drupal.dialog']['dependencies'][] = 'seven/seven.drupal.dialog';
+  if ($extension == 'classy' && isset($libraries['dialog'])) {
+    unset($libraries['dialog']['css']['component']['css/components/dialog.css']);
+    $libraries['dialog']['dependencies'][] = 'seven/seven.drupal.dialog';
   }
 }