Your markup looks correct. What email client are you testing this on? Not all email clients support the use of web fonts. I don’t think Gmail supports web fonts.
Try adding a div with the class you are wanting to style directly to the invite template and then adding styles to the class to the Email Style / CSS editor. Discourse will substitute the styles you add there directly into the div that you target with a class. For example, with the Invite Mailer template customized to:
<div class="invite">
%{inviter_name} invited you to a discussion
> **%{topic_title}**
>
> %{topic_excerpt}
at
> %{site_title} -- %{site_description}
If you're interested, click the link below:
%{invite_link}
</div>
and the following added to Email Styles / CSS
.invite {
border: 1px solid red;
}
Invites have a red border around the content.