To be able to change the font of your email templates, you need to request to enable an HTML editor for your account. Contact us in Live Chat or via email: firstname.lastname@example.org.
When it's enabled, the source code icon will appear in the toolbar:
Here's an example of how to wrap your text to apply the font:
<span style="font-size: 11px; font-family: georgia; color: black;"> your email body text here </span>