Dan, you can use CSS to define your HTML tags in Marketo emails. You would define your CSS in the email template. Then when you create an email based on that template, in the Email Designer you can access the HTML of the editable sections and add in-line CSS if you want.
The
Emails deep dive help article has a section of links to help articles about Templates. Also if you search for "Email Template" in the Community, there are several help articles for Marketo's pre-defined templates, which you can download. You can also use the Import Program function to download a large number of pre-created Responsive Email Templates that display well on many mobile devices.