diff --git a/core/modules/system/tests/themes/test_basetheme/base-add.css b/core/modules/system/tests/themes/test_basetheme/base-add.css
new file mode 100644
index 0000000..fc02680
--- /dev/null
+++ b/core/modules/system/tests/themes/test_basetheme/base-add.css
@@ -0,0 +1,5 @@
+/* Thanks to http://jonathandean.com/2013/01/showing-the-current-breakpoint-name-when-testing-responsive-designs-using-only-css/ */
+body:after {
+  content: 'Showing test_basetheme';
+  font-weight: bold;
+}
diff --git a/core/modules/system/tests/themes/test_subtheme/css/sub-add.css b/core/modules/system/tests/themes/test_subtheme/css/sub-add.css
new file mode 100644
index 0000000..a1dad90
--- /dev/null
+++ b/core/modules/system/tests/themes/test_subtheme/css/sub-add.css
@@ -0,0 +1,5 @@
+/* Thanks to http://jonathandean.com/2013/01/showing-the-current-breakpoint-name-when-testing-responsive-designs-using-only-css/ */
+body:after {
+  content: 'Showing test_subtheme';
+  font-weight: bold;
+}