/**
 * Load Font Awesome's CSS from CDN
 *
 * @param  string                $stylesheet_uri Icon type's stylesheet URI.
 * @param  string                $icon_type_id   Icon type's ID.
 * @param  Icon_Picker_Type_Font $icon_type      Icon type's instance.
 *
 * @return string
 */
function myprefix_font_awesome_css_from_cdn( $stylesheet_uri, $icon_type_id, $icon_type ) {
	if ( 'fa' === $icon_type_id ) {
		$stylesheet_uri = sprintf(
			'https://maxcdn.bootstrapcdn.com/font-awesome/%s/css/font-awesome.min.css',
			$icon_type->version
		);
	}

	return $stylesheet_uri;
}
add_filter( 'icon_picker_icon_type_stylesheet_uri', 'myprefix_font_awesome_css_from_cdn', 10, 3 );