Set a custom theme to be used by Symfony Mailer

symfony_mailer.mailer_policy._.yml

configuration:
  email_theme:
    theme: my_custom_theme

Prerequisite:

Installed and enabled the Symfony Mailer module.

You have multiple themes enabled in your drupal page and need to select the theme that includes custom templates for send emails.

Configure Symfony Mailer to use templates coming from your custom theme.

Go to `/admin/config/system/mailer` and edit the policy. There you can switch the theme that is used for sending your e-mails.