Bellows Control Panel > Main Configuration > Font
The Font Family setting allows you to set the font-family for the menu.
It is important to note that this does not actually load any font assets. So you should only add font families that you expect your users to have on their devices (standard font stacks) or @font-face fonts that you have already loaded on your site through other means.
In other words, this just sets a CSS property, it doesn’t actually deliver the font itself.