HTML emails are a strange beast. You need to inline all the CSS, and there are edge cases (Outlook!) to take care of.
To get going fast here are resources I found useful.
https://reallygoodemails.com/emails/activate-your-spothero-account - samples people have uploaded
https://stripo.email/ Build your own and templates.
https://htmlemail.io/inline/ - useful to check if it renders!
I used an email template from stripo as a baseline and modified the underlying html.
I’m happy to have a good looking email