i don't think there is a forum option to set a default font. you might be able to do it for your whole computer though.
No there isn't, the only person that can add this option is shawn and it looks like he's run of to barbados or something lol, so it won't be happening. If it can be done there may be a way do to it locally on your system but the forum skin itself sets the font, so any changes you make locally may just be overidden. So it'll likely be more effort than it's worth, i'd give up on the idea tbh, everyone else puts up with it