How to customise/override `email-frame.less` or inject styles inside the message iframe?

The default email-frame.less sets some pretty opinionated styles for the rendered content of email messages themselves, including hard-coding the use of the Nylas-Pro font:

I’m working on a theme that uses the system-ui font throughout, instead of Nylas-Pro, but I can’t see a way to customise or override this line, which gets loaded inside the message iframe. My theme’s index.less isn’t loaded inside the iframe – it looks like the only thing that gets injected is app/static/style/email-frame.less.

Has anyone found a way to customise the styles inside the message iframe?