The styling of the emails is necessarily a completely seperate thing from the rest of the theme, as they are displayed in two different contexts, and styling emails is notoriously tricky for various reasons.
Mostly Moodle leaves the emails alone, but there is a template in lib/templates/email_html.mustache that a theme developer could override in a theme to change the font styling for the whole email. However, I'd proceed with caution, for example putting too much styling in here can get your emails blacklisted as spam
, or render them unreadable in various different email clients unless you pay attention to a lot of obscure rules. Web design for emails is about 10 years behind the rest of the web and stuff designers take for granted may not be available or may break things.