How do you override a module's template from another module?

Question

I'm trying to override an email template of the webform module. The project is headless so we don't have a custom theme, and we'd like to stick with the Adminimal contrib theme. However, we do have a custom module named app in web/modules/app.

Here are the default template suggestions shown in the email source:

<!-- THEME DEBUG -->
<!-- THEME HOOK: 'webform_email_html' -->
<!-- FILE NAME SUGGESTIONS:
   * webform-email-html--contact--email--email-notification.html.twig
   * webform-email-html--contact--email.html.twig
   * webform-email-html--contact.html.twig
   x webform-email-html.html.twig
-->
<!-- BEGIN OUTPUT from 'modules/contrib/webform/templates/webform-email-html.html.twig' -->

If I just create a web/modules/app/templates/webform-email-html.html.twig file (or any of the other suggestions), the template override doesn't work: the template of the webform module is used.

Answer 1

The only way I found to get template override to work is to:

1. add a template suggestion:

   // in web/modules/app/app.module
   function app_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
     $suggestions[] = $hook . '--' . 'custom';
   }
   

2. force the template used to render the webform_email_html--custom template suggestion:

   // in web/modules/app/app.module
   function app_theme($existing, $type, $theme, $path) {
     return [
       'webform_email_html--custom' => [
         'template' => 'webform-email-html--custom',
       ],
     ];
   }
   

3. create the web/modules/app/templates/webform-email-html.html.twig file

I'm not marking this answer as accepted on purpose: please tell me there's a better way, this feels overly complicated!