diff --git a/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php b/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php
index f50dc8b..5ad4015 100644
--- a/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php
+++ b/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php
@@ -194,6 +194,35 @@ protected function testLoadingWithoutInternalButtons() {
     $this->assertTrue(isset($editor_settings['disallowedContent']));
   }
 
+  /**
+   * Tests loading of theme's CKEditor stylesheets defined in the .info file.
+   */
+  function testExternalStylesheets() {
+    // Case 1: Install theme which has an absolute external CSS URL.
+    \Drupal::service('theme_handler')->install(['test_ckeditor_stylesheets_external']);
+    \Drupal::service('theme_handler')->setDefault('test_ckeditor_stylesheets_external');
+    $expected = [
+      'https://fonts.googleapis.com/css?family=Open+Sans',
+    ];
+    $this->assertIdentical($expected, _ckeditor_theme_css('test_ckeditor_stylesheets_external'));
+
+    // Case 2: Install theme which has an external root-relative CSS URL.
+    \Drupal::service('theme_handler')->install(['test_ckeditor_stylesheets_external_root_relative']);
+    \Drupal::service('theme_handler')->setDefault('test_ckeditor_stylesheets_external_root_relative');
+    $expected = [
+      '//fonts.googleapis.com/css?family=Open+Sans',
+    ];
+    $this->assertIdentical($expected, _ckeditor_theme_css('test_ckeditor_stylesheets_external_root_relative'));
+
+    // Case 3: Install theme which has a relative CSS URL.
+    \Drupal::service('theme_handler')->install(['test_ckeditor_stylesheets_relative']);
+    \Drupal::service('theme_handler')->setDefault('test_ckeditor_stylesheets_relative');
+    $expected = [
+      '0' => 'core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/css/yokotsoko.css',
+    ];
+    $this->assertIdentical($expected, _ckeditor_theme_css('test_ckeditor_stylesheets_relative'));
+  }
+
   protected function getThingsToCheck() {
     $settings = $this->getDrupalSettings();
     return array(
diff --git a/core/modules/system/tests/themes/test_ckeditor_stylesheets_external/test_ckeditor_stylesheets_external.info.yml b/core/modules/system/tests/themes/test_ckeditor_stylesheets_external/test_ckeditor_stylesheets_external.info.yml
new file mode 100644
index 0000000..ff65dbe
--- /dev/null
+++ b/core/modules/system/tests/themes/test_ckeditor_stylesheets_external/test_ckeditor_stylesheets_external.info.yml
@@ -0,0 +1,9 @@
+name: Test External CKEditor stylesheets
+type: theme
+description: 'A theme that uses an external webfont stylesheet.'
+version: VERSION
+base theme: false
+core: 8.x
+
+ckeditor_stylesheets:
+  - https://fonts.googleapis.com/css?family=Open+Sans
diff --git a/core/modules/system/tests/themes/test_ckeditor_stylesheets_external_root_relative/test_ckeditor_stylesheets_external_root_relative.info.yml b/core/modules/system/tests/themes/test_ckeditor_stylesheets_external_root_relative/test_ckeditor_stylesheets_external_root_relative.info.yml
new file mode 100644
index 0000000..397b936
--- /dev/null
+++ b/core/modules/system/tests/themes/test_ckeditor_stylesheets_external_root_relative/test_ckeditor_stylesheets_external_root_relative.info.yml
@@ -0,0 +1,9 @@
+name: Test External CKEditor stylesheets
+type: theme
+description: 'A theme that uses an external webfont stylesheet.'
+version: VERSION
+base theme: false
+core: 8.x
+
+ckeditor_stylesheets:
+  - //fonts.googleapis.com/css?family=Open+Sans
diff --git a/core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/css/yokotsoko.css b/core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/css/yokotsoko.css
new file mode 100644
index 0000000..1958f4d
--- /dev/null
+++ b/core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/css/yokotsoko.css
@@ -0,0 +1,4 @@
+/**
+ * @file
+ * Test CSS asset file for test_ckeditor_stylesheets_relative.theme.
+ */
diff --git a/core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/test_ckeditor_stylesheets_relative.info.yml b/core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/test_ckeditor_stylesheets_relative.info.yml
new file mode 100644
index 0000000..e3003af
--- /dev/null
+++ b/core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/test_ckeditor_stylesheets_relative.info.yml
@@ -0,0 +1,9 @@
+name: Test External CKEditor stylesheets
+type: theme
+description: 'A theme that uses an external webfont stylesheet.'
+version: VERSION
+base theme: false
+core: 8.x
+
+ckeditor_stylesheets:
+  - css/yokotsoko.css