To change the default font in the editor you can use cascading style sheets e.g.:

body { font-family:SansSerif; }

or e.g.

p { font-family:SansSerif; }

To load this style sheet into the applet, you can use the "setstylesheet" or "setstylesheeturl" Java Script API function (for more information, please review the edit-on Pro integration manual, chapter 2.4 "Java Script API") or the "stylesheeturl" Applet parameter, chapter 2.1 "Applet Parameter API".