Tyler Staples
--- a/core/modules/system/src/Form/ThemeExperimentalConfirmForm.php
+++ b/core/modules/system/src/Form/ThemeExperimentalConfirmForm.php
        '@theme' => $get_label($theme),
        // It is safe to implode this because theme names are not translated
        // markup and so will not be double-escaped.
-        '@required' => implode(', ', array_map($get_label, $dependencies)),
+        '@required' => (function ($items) {
+          $count = count($items);
+          if ($count <= 1) {
+            return implode('', $items);
+          }
+          if ($count == 2) {
+            return implode(' and ', $items);
+          }
+          $last = array_pop($items);
+          return $items ? implode(', ', $items) . ', and ' . $last : $last;
+        })(array_map($get_label, $dependencies)),