This can be achieved by adding a CSS override in your theming to the appropiate class (something with .mceContentBody).
However, i strongly recommend against using a default font such as Lucinda Sans Unicode, because this is not a web core font, for which reason it might look different on different machines and different browser, which usually results in numberous other styling problems with text that don't fit their boxes etc. So if you insist in change the default font, use one of those: