Get rid of duplicate CSS for template preview

We can reuse the same CSS we pass to editor
[MAILPOET-6249]

packages/php/email-editor/tests/integration/_bootstrap.php

@@ -109,6 +109,7 @@ abstract class MailPoetTest extends \Codeception\TestCase\Test { // phpcs:ignore
     $container->set(TemplatePreview::class, function ($container) {
       return new TemplatePreview(
         $container->get(ThemeController::class),
+        $container->get(SettingsController::class),
         $container->get(Templates::class),
       );
     });