diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index f9849c7..c47bf8b 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -160,6 +160,10 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
 
     $values['path'] = $theme_path;
     $values['name'] = $theme->getName();
+    // If no theme could be negotiated, or if the negotiated theme is not within
+    // the list of installed themes, getActiveThemeByName() will assign 'core'
+    // as theme name. Mark this theme as disabled.
+    // @see getActiveThemeByName()
     $values['status'] = $values['name'] != 'core';
 
     // Prepare stylesheets from this theme as well as all ancestor themes.