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

services:
  postoffice_inline_styles.message_subscriber:
    class: \Drupal\postoffice_inline_styles\EventSubscriber\MessageSubscriber
    arguments:
      - '@asset.resolver'
    tags:
      - { name: event_subscriber }