Load Font Awesome using MaxCDN. Source: http://www.bootstrapcdn.com/fontawesome/

load-font-awesome-cdn-conditionally.php

<?php

//* Load Font Awesome using MaxCDN conditionally
add_action('wp_enqueue_scripts', 'customprefix_enqueue_font_awesome');
function customprefix_enqueue_font_awesome() {
    if (is_category() || is_home()) {
        wp_enqueue_style('font-awesome', '//maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css');
    }
}

load-font-awesome-cdn.php

<?php

//* Load Font Awesome using MaxCDN
add_action('wp_enqueue_scripts', 'customprefix_enqueue_font_awesome');
function customprefix_enqueue_font_awesome() {
    wp_enqueue_style('font-awesome', '//maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css');
}