The default is: 'Lucida Grande', 'Lucida Sans Unicode', Verdana, Helvetica, sans-serif, 'GNU Unifont'

Put any font name in here, and if it's installed on your computer, it'll work for you. If you use several different devices, specify some fall-back fonts, with commas in between the names, in case one of your devices doesn't have the first font.

You can use either single or double quotation marks around fonts with multi-word names, e.g. "Lucida Grande" or 'Lucide Sans Unicode'.