Adding Font Icon Library

function my_prefix_add_font_icon( $list_fonts ){
    // add new
    $list_fonts['my_library'] =  array(
        'name' => __('My Font Icons', 'customify'),
        'icons' => array(
            // Your icon classes here
        ),
        'url' => 'http://domain.com/path_to_css_file.css',
        'class_config' => 'fa __icon_name__' // __icon_name__ will replace by icon class name
    );
    return $list_fonts;
}
add_filter( 'customify/customizer/font_icons', 'my_prefix_add_font_icon' );