You like a theme but it's referencing a Google font you want to switch out. Here's the simplest way to do it, by keeping the .class names but switching out the Google Font URL.

/** ******** ******** ******** ******** ******** ******** ******** ******** 
* TITLE:        Add Font
* DESCRIPTION: Add Specific Google Font
*/

function add_fonts() {
	// Styles Format:  wp_enqueue_style($handle, $src, $deps, $ver, $media);
        wp_register_style('open-sans', 'http://fonts.googleapis.com/css?family=Bree+Serif:400,300,700', array(), false, 'all');
        wp_enqueue_style( 'open-sans');
}
add_action('wp_enqueue_scripts', 'add_fonts', 100);