You need first to include your fonts. Eater by using google fonts or by putting your font sources within template /css folder then including them using @fontface css rules.
If you use google fonts then you need to put link to font sources google generates for you inside <head> setion in index.php of template. For other way put .eot's and .ttf's inside template /css folder, then use @fontface at start of template_css.css to include them.
After you have this fonts hooked you can replace all font-family: properties you find in template_css.css, layout.css, /styles/style[N].css whit your fonts
