Go to the Styles panel (Ctrl-Alt-Shift-S), click the Options button at
the lower right, set the dropdowns to "All Styles" and "Alphabetical,"
ok, locate the character style called Line Number, right-click, choose
Modify, and change the font.
You'll probably want to change "All Styles" back to "In Use" or "In
This Document" before you go back to work!
