Change font in macOS Messages app

Back in Mojave I was able to change the default font in the Messages app, I think with a setting in System Preferences > Accessibility.

After upgrading to Big Sur that font change remained, but for some reason it got reset today, right after I had to use Activity Monitor to quit Notifications (because the Notifications pane was stuck open and was beachballing). https://9apps.ooo/

Now I’m back to the standard font, which I don’t like, and I don’t see any way to change the font, either in Messages’ preferences or in Big Sur’s Accessibility PrefPane.

Am I missing something? Anyone know how I can choose a different font again for macOS Messages?