@skyoung: Sorry, I am snot sure which heading you want to change. So, I am giving you css to change font family and size of site title and page title. Just change the size and family as per your need and then add it in “Appearance => Customize => Theme Options => Custom CSS Options” box:
/* Site Title Font */
.site-title {
font-family: "Droid Sans",sans-serif;
font-size: 38px;
}
/* Page-Post Font */
.entry-title,
#promotion-message h2,
#featured-content #featured-heading {
font-family: "Droid Sans",sans-serif;
font-size: 30px;
}
Note: when you change the font family, make sure you change with Web safe font or you need to add with font plugins.