You can also do this with any theme if you setup a CSS Snippet:

// This is the one that works for me
.markdown-source-view.mod-cm6 .cm-scroller {
    font-family: monospace;
}

// You may also want to do this one
.markdown-source-view {
    font-family: monospace;
}

Then reading view will use whatever font you’ve configured in the settings.

@berryjam I’ve tried to sign up for Lemmy twice and it’s failed both times. Please tell them they can use different fonts for live preview and for reading mode with the Minimal theme and the Minimal Theme Settings plugin.