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.
Reference:
Excerpt of the user's manual:
File extension
*.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.