My understanding is that if you want to use something besides the default font, you have to select it each time you post. I see choices in Edit Options for "skins", for showing or hiding signatures, for what text editor to use when posting, and for the order in which posts are displayed, but nothing about fonts.

If you don't see the "Fonts" and "Sizes" selectors above the text box when you're posting, try clicking on the "Go Advanced" button.