diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index 6f35ca373072..613ab1b7bd95 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -91,7 +91,7 @@ function _ckeditor_theme_css($theme = NULL) {
       }
     }
     if (isset($info['base theme'])) {
-      $css = array_merge(_ckeditor_theme_css($info['base theme'], $css));
+      $css = array_merge(_ckeditor_theme_css($info['base theme']), $css);
     }
   }
   return $css;
diff --git a/core/modules/ckeditor/src/Tests/CKEditorTest.php b/core/modules/ckeditor/src/Tests/CKEditorTest.php
index 46390a4a2099..9133ee00512e 100644
--- a/core/modules/ckeditor/src/Tests/CKEditorTest.php
+++ b/core/modules/ckeditor/src/Tests/CKEditorTest.php
@@ -262,7 +262,11 @@ class CKEditorTest extends KernelTestBase {
     $expected[] = file_create_url('core/modules/ckeditor/tests/modules/ckeditor_test.css');
     $this->assertIdentical($expected, $this->ckeditor->buildContentsCssJSSetting($editor), '"contentsCss" configuration part of JS settings built correctly while a hook_ckeditor_css_alter() implementation exists.');
 
-    // @todo test coverage for _ckeditor_theme_css(), by including a custom theme in this test with a "ckeditor_stylesheets" entry in its .info file.
+    // Enable the Bartik theme, which specifies a CKEditor stylesheet.
+    \Drupal::service('theme_handler')->install(['bartik']);
+    \Drupal::config('system.theme')->set('default', 'bartik');
+    $expected[] = file_create_url('core/themes/bartik/css/ckeditor-iframe.css');
+    $this->assertIdentical($expected, $this->ckeditor->buildContentsCssJSSetting($editor), '"contentsCss" configuration part of JS settings built correctly while a theme providing a CKEditor stylesheet exists.');
   }
 
   /**