diff --git a/core/modules/comment/comment.libraries.yml b/core/modules/comment/comment.libraries.yml
index d19ff93..78c4ff0 100644
--- a/core/modules/comment/comment.libraries.yml
+++ b/core/modules/comment/comment.libraries.yml
@@ -7,12 +7,6 @@ drupal.comment:
     - core/drupal
     - core/drupal.form
 
-drupal.comment.threaded:
-  version: VERSION
-  css:
-    theme:
-      css/comment.theme.css: {}
-
 drupal.comment-by-viewer:
   version: VERSION
   js:
diff --git a/core/modules/comment/css/comment.theme.css b/core/modules/comment/css/comment.theme.css
deleted file mode 100644
index c702161..0000000
--- a/core/modules/comment/css/comment.theme.css
+++ /dev/null
@@ -1,16 +0,0 @@
-
-/**
- * @file
- * Basic styling for comment module.
- */
-
-/**
- * Indent threaded comments.
- */
-.indented {
-  margin-left: 25px; /* LTR */
-}
-[dir="rtl"] .indented {
-  margin-left: 0;
-  margin-right: 25px;
-}
diff --git a/core/modules/comment/src/CommentViewBuilder.php b/core/modules/comment/src/CommentViewBuilder.php
index 33187d2..bd203b5 100644
--- a/core/modules/comment/src/CommentViewBuilder.php
+++ b/core/modules/comment/src/CommentViewBuilder.php
@@ -183,7 +183,6 @@ protected function alterBuild(array &$build, EntityInterface $comment, EntityVie
 
       // Add indentation div or close open divs as needed.
       if ($build['#comment_threaded']) {
-        $build['#attached']['library'][] = 'comment/drupal.comment.threaded';
         $prefix .= $build['#comment_indent'] <= 0 ? str_repeat('</div>', abs($build['#comment_indent'])) : "\n" . '<div class="indented">';
       }
 
diff --git a/core/themes/classy/classy.libraries.yml b/core/themes/classy/classy.libraries.yml
index 116c86b..3c70848 100644
--- a/core/themes/classy/classy.libraries.yml
+++ b/core/themes/classy/classy.libraries.yml
@@ -3,3 +3,9 @@ base:
   css:
     theme:
       css/layout.css: {}
+
+drupal.comment.threaded:
+  version: VERSION
+  css:
+    theme:
+      css/comment/comment.theme.css: {}
diff --git a/core/themes/classy/css/comment/comment.theme.css b/core/themes/classy/css/comment/comment.theme.css
new file mode 100644
index 0000000..c702161
--- /dev/null
+++ b/core/themes/classy/css/comment/comment.theme.css
@@ -0,0 +1,16 @@
+
+/**
+ * @file
+ * Basic styling for comment module.
+ */
+
+/**
+ * Indent threaded comments.
+ */
+.indented {
+  margin-left: 25px; /* LTR */
+}
+[dir="rtl"] .indented {
+  margin-left: 0;
+  margin-right: 25px;
+}
diff --git a/core/themes/classy/templates/content/comment.html.twig b/core/themes/classy/templates/content/comment.html.twig
index 1d56d08..1decd44 100644
--- a/core/themes/classy/templates/content/comment.html.twig
+++ b/core/themes/classy/templates/content/comment.html.twig
@@ -63,6 +63,7 @@
  * @see template_preprocess_comment()
  */
 #}
+{{ attach_library('classy/drupal.comment.threaded') }}
 {%
   set classes = [
     'comment',