If you use swiftmailer + simplenews, you may want to send html email with custom css.

To achieve that,you have to customise 2 twig templates.

The first template is the default template that is provided by swiftmailer: simplenews-newsletter-body.html.twig.

You can use this template to build your own email body with content you like to use. For example, in the template below, an hero image is inserted using table layout.