Font styling is found in the style.css around line 1950 and looks like this:
.nav-menu li {
display: inline-block;
position: relative;
background:none;
padding-left:0;
font-family: ubuntu,sans-serif;
font-size: 15px;
font-size: 0.938rem;
line-height: 1.42857143;
}
However, I see something that needs to be removed on my next update but you can do this to the core theme style.css….remove this:
font-size: 15px;
from this around line 1961:
.nav-menu li a {
color: #e4e6eb;
display: block;
font-size: 15px;
line-height: 1;
padding: 10px 12px;
text-decoration: none;
}
For your child theme, the one you would create a custom css for is the .nav-menu li