diff --git a/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
index 0575c7b..dcb1a2f 100644
--- a/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.info.yml
@@ -6,4 +6,4 @@ core: 8.x
 libraries:
   - test_basetheme/global-styling
 stylesheets-remove:
-  - @theme_test/css/base-remove.css
+  - '@theme_test/css/base-remove.css'
diff --git a/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
index 2f96f2c..6883e5a 100644
--- a/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
+++ b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
@@ -7,5 +7,5 @@ base theme: test_basetheme
 libraries:
   - test_subtheme/global-styling
 stylesheets-remove:
-  - @theme_test/css/sub-remove.css
-  - @test_basetheme/base-add.sub-remove.css
+  - '@theme_test/css/sub-remove.css'
+  - '@test_basetheme/base-add.sub-remove.css'
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.info.yml b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
index 0c3505c..7ddcb2d 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.info.yml
+++ b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
@@ -15,7 +15,7 @@ version: VERSION
 base theme: classy
 core: 8.x
 stylesheets-remove:
-  - @system/css/system.module.css
+  - '@system/css/system.module.css'
 regions:
   content: Content
   left: Left
diff --git a/core/themes/bartik/bartik.info.yml b/core/themes/bartik/bartik.info.yml
index 6f98537..3c02894 100644
--- a/core/themes/bartik/bartik.info.yml
+++ b/core/themes/bartik/bartik.info.yml
@@ -6,7 +6,7 @@ package: Core
 version: VERSION
 core: 8.x
 stylesheets-remove:
-  - @classy/css/layout.css
+  - '@classy/css/layout.css'
 libraries:
   - bartik/global-styling
 ckeditor_stylesheets:
diff --git a/core/themes/seven/seven.info.yml b/core/themes/seven/seven.info.yml
index 57bc632..8ee0914 100644
--- a/core/themes/seven/seven.info.yml
+++ b/core/themes/seven/seven.info.yml
@@ -10,7 +10,7 @@ libraries:
  - seven/global-styling
 stylesheets-remove:
   - core/assets/vendor/jquery.ui/themes/base/dialog.css
-  - @classy/css/layout.css
+  - '@classy/css/layout.css'
 quickedit_stylesheets:
   - css/components/quickedit.css
 regions: