postoffice-1.0.x-dev/extensions/postoffice_compat_theme/postoffice_compat_theme.services.yml

services:
  postoffice_compat_theme.mail:
    public: false
    class: Drupal\postoffice_compat_theme\Mail\ThemeMailManager
    decorates: plugin.manager.mail
    arguments:
      - '@postoffice_compat_theme.mail.inner'
      - '@config.factory'
      - '@theme.manager'
      - '@theme.initialization'