In the theme’s style.css, you will see around line 154 this:
#nav-wrapper {
background-color:#78a5b6;
border-radius:60px 60px 0 0;
-webkit-border-radius: 60px 60px 0px 0px;
height:37px;
text-align:center;
font:normal 14px open_sans, Arial, sans-serif;
font-size: 0.875rem;
color:#fff;
position:relative;
z-index:1000;
}
I’m using rem as a font size that overrides the px, but if you want to use just px, then remove the rem font size line. Then of course, the font you see is open_sans….but you can change that there.