It's not in preferences-dialog, right. But you can define this behavior directly in the file prefs.js, see:
http://kmeleon.freewebsites.com/kmeleon-manual/configs.html#CONFIG-PREFS
user_pref("browser.display.use_document_fonts", 0);
// 0 = never, 1 = quick, 2 = always
user_pref("browser.display.use_document_colors", false);
user_pref("browser.display.use_system_colors", false);
user_pref("browser.display.foreground_color", "#000000"); //black
user_pref("browser.display.background_color", "#ffffff"); //white
user_pref("font.default", "sans-serif");
user_pref("font.minimum-size.x-western", 14);
user_pref("font.name.sans-serif.x-western", "Verdana");
user_pref("font.name.serif.x-western", "Georgia");
user_pref("font.size.fixed.x-western", 17);
user_pref("font.size.variable.x-western", 17);