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
@@ -53,12 +53,7 @@
  */
 function template_preprocess_admin_block_content(&$variables) {
   if (!empty($variables['content'])) {
-    // Check if compact mode is turned on.
-    // NOTICE: system_admin_compact_mode() seems to return a string
-    // and not a boolean.
-    $compact = system_admin_compact_mode();
-    $variables['compact'] = ($compact) ? TRUE : NULL;
-
+    $variables['compact'] = system_admin_compact_mode();
     foreach ($variables['content'] as $key => $item) {
       $variables['content'][$key]['link'] = \Drupal::linkGenerator()->generateFromUrl($item['title'], $item['url']);
       if (!$compact && isset($item['description'])) {
diff -u b/core/modules/system/templates/admin-block-content.html.twig b/core/modules/system/templates/admin-block-content.html.twig
--- b/core/modules/system/templates/admin-block-content.html.twig
+++ b/core/modules/system/templates/admin-block-content.html.twig
@@ -9,7 +9,7 @@
  *   contain the keys 'title', 'link_path', and 'localized_options', which are
  *   passed to l(). A 'description' key may also be provided.
  * - attributes: HTML attributes to be added to the element.
- * - compact: True if compact mode is turned on.
+ * - compact: Boolean indicating whether compact mode is turned on or not.
  *
  * @see template_preprocess_admin_block_content()
  *
diff -u b/core/modules/system/templates/system-themes-page.html.twig b/core/modules/system/templates/system-themes-page.html.twig
--- b/core/modules/system/templates/system-themes-page.html.twig
+++ b/core/modules/system/templates/system-themes-page.html.twig
@@ -15,8 +15,10 @@
  *     - description: Description of the theme.
  *     - name: Theme name.
  *     - version: The theme's version number.
- *     - is_default: If the theme is the default theme.
- *     - is_admin: If the theme is the admin theme.
+ *     - is_default: Boolean indicating whether the theme is default theme or
+ *       not.
+ *     - is_admin: Boolean indicating whether the theme is the admin theme or
+ *       not.
  *     - notes: Identifies what context this theme is being used in, e.g.,
  *       default theme, admin theme.
  *     - incompatible: Text describing any compatibility issues.