The are many email platforms that accept HTML/CSS. I would recommend looking into Postmark. Their documentation is quite good. I believe there’s also a Bubble plugin or 2 if you want an even simpler implementation.
You can pass full HTML code directly into Sendgrid (just set up a blank template with a single handlebar to receive the HTML)… so you can create your email HTML wherever you like then just use Sendgrid to deliver the email.