Just like the header background color you asked earlier, this is matter of CSS. With the same steps as I described, you just need to create own CSS file and overwrite any style definitions there.
#mollify {} #mollify, a.button, input, .mollify-popupmenu, .mollify-popup, .mollify-itemcontext-popover {
font-size: 12px;
font-family: 'proxima-nova', 'MollifyPlainText', "Lucida Grande", "Trebuchet MS", Lucida, Verdana, sans-serif;
}
li.dropdown-item, .dropdown-submenu {
font-family: 'proxima-nova', 'MollifyPlainText', "Lucida Grande", "Trebuchet MS", Lucida, Verdana, sans-serif;
font-size: 14px;
}
If you want to change font _everywhere_, you can simply copy&paste these into your own CSS and change the font definition.
If you want to change font in only specific area/element, then it's something you have to figure out with, for example, element inspector in Chrome etc, where you can see the element css classes etc.