Fix & test

Closes #2971692

modules/private_message_notify/tests/src/Kernel/PrivateMessageNotifierTest.php

@@ -48,6 +48,13 @@ class PrivateMessageNotifierTest extends KernelTestBase {
 
     $this->config('system.site')->set('name', 'Aerosmith')->save();
 
+    // Theme us needed for mail body rendering.
+    $this->container->get('theme_installer')->install(['stark']);
+    $this->config('system.theme')
+      ->set('admin', 'stark')
+      ->set('default', 'stark')
+      ->save();
+
     $this->installEntitySchema('user');
     $this->installSchema('user', ['users_data']);
     $this->createTestingUsers();
@@ -90,6 +97,7 @@ class PrivateMessageNotifierTest extends KernelTestBase {
     $messageTemplate->set('text', $text)->save();
 
     $this->config('mailsystem.settings')
+      ->set('theme', 'stark')
       ->set('defaults', [
         'sender' => 'test_mail_collector',
         'formatter' => 'symfony_mailer_lite',