The fonts for the headings are found in the theme’s style.css file around line 231
h1, h2, h3, h4, h5, h6{
font-family: open_sans, sans-serif;
font-weight:400;
color:#333;
margin-bottom:16px;
margin-bottom:1rem;
}
Then below that code you will see the different heading selectors as h1, h2, etc:
h1 {
font-size:23px;
font-size:1.438rem;
}
You will notice I used two font sizes…px which is common, and then rem which is a relative size and used for the more modern browsers. You can remove the rem lines if you want to just use px instead. But to help give you a reference, this online tool is a conversion from px to em (also rem) and vice versa: http://www.pxtoem.com