Custom Template

I found how it works.

Create a file "mail.css" in your X theme's css folder:
themes/contrib/X/css/mail.css

Add this line at the bottom of your theme's X.libraries.yml file:
-------------
## Symphony Mailer
email:
css:
theme:
css/mail.css: { preprocess: false }
-------

Configure the mail policy with your X theme, here: /admin/config/system/mailer

Automatically closed - issue fixed for 2 weeks with no activity.