I recommend to make your customizations in the Custom Head Styles administration, because if someday you want to update your theme with our latest version you will lose your changes. The style you need to put in there in order to overwrite the fonts is :
.justAFixingDivBefore, div.header-menu,
.header-menu > ul > li > a, .header-menu > ul > li > span,
div.carousel-title span {
font-family: '';
}
And put the name of the font you want to use in the quotation marks.
Hope that helps