You’re right: the TextFont attribute of the built-in Markdown prototype is Ideal Sans, so I modified the built-in prototype to create a custom prototype that has the typeface I like and that is available to all my Tinderbox files as described here.
I still don’t understand why the typeface doesn’t change to Ideal Sans when I apply the built-in Markdown prototype; it only changes when I apply both the Markdown prototype and the Markdown highlighter.
In any case, thank you for the prompt and helpful reply.
Best regards,
Enrico