Don't store the font-family preference with plain text messages.

Dynamically generate the CSS style for <pre> elements
for inclusion in the HTML <head> element when messages
are displayed.

This permits a user to change their font-family preference
for plain text messages and see the results immediately.

Obviously any old locally-stored messages that had their
font-family stored with them will continue to display using
that font-family, irrespective of the user's current
preference setting.