diff -u b/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
--- b/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -346,7 +346,7 @@
     'extension--theme',
   ];
 
-  // Build a list of theme names for all known themes
+  // Build a list of theme names for all known themes.
   $theme_infos = [];
   foreach ($variables['theme_group_titles'] as $state => $title) {
     foreach ($theme_groups[$state] as $theme) {