It may be over spec for you. But, "Notification Pro" has its feature (Conversion to multipart email + Customizable email template). The default template is the template of popular mailchimp. If you have HTML and CSS knowledge, you are free to customize HTML email.
Excerpt of the user's manual:
*.txt: Template for plain text part
*.html: Template for HTML text part
*.css: Style for HTML text part
File explanation of main part
main.html: Template of basic part applied to all emails
style.css: Style applied to all emails
Conversion rule of main part
[title]: Converted to “mail title”
[style]: Converted to contents in “style.css”
[site_url]: Converted to “site URL”
[site_title]: Converted to “site title”
[headerContent]: Converted to “images/eyecatch_600px.png”
This is commented with initial state.
[body]: Converted to email content
[unsubscribe]: Converted to “unsubscribe link”
This is displayed only in the case of email sent by NP.