In the Windows desktop client of Slack is it possible to change the font of the messages?

This can now be done with the /slackfont command: e.g. /slackfont Times New Roman

There was a discussion on Twitter with the Slack developers about being able to change fonts for accessibility reasons: https://twitter.com/manchicken/status/718501914661548032.

Unfortunately, changing the font (other than switching to the built-in compact theme) is not a feature.