I've never customized it so I have no idea whether it's a scary mess that sort of works but not really because you're taking it outside its comfort zone, or whether it's all pretty easy. I guess I'll have to play around with it some when the time comes.
Customizing it with less is really pretty straightforward.
You can also generate custom css from the website. Just click the customize tab in the topbar. You can bump up @baseFontSize. 16px works very well. You might also want to increase the @baseLineHeight a bit as well.
Thanks!