An email can be added to the HTML document and sent to the client. Those HTML styles would be then rendered as the styles of the email itself.
For example, you can think of the following HTML markup
<div style="font-family: 'Segoe UI'; padding: 10px; color: #cecece">
Hello, this is the email body.
.. you can pass that entire code, as a string, to the body of the email. Once the email client would get it, it will render it as the styles would guide. There is no other special language to learn while designing the emails. You can design a web page, add data to it using the variables, fill the body and elements and just send it.