--- /dev/null
+<?php
+
+/**
+ * @file
+ * Test module.
+ */
+
+use Drupal\Core\Extension\Extension;
+
+/**
+ * Implements hook_system_info_alter().
+ */
+function theme_page_test_system_info_alter(&$info, Extension $file, $type) {
+ // Make sure that all themes are visible on the Appearance form.
+ if ($type == 'theme') {
+ unset($info['hidden']);
+ }
+}