siepaw

@george J, last update at now, add opensans font for form elements in media modal window:

// add custom css
$custom_css = 'body, #wpadminbar *:not([class="ab-icon"]), .wp-core-ui, .media-frame input[type="text"], .media-frame input[type="password"], .media-frame input[type="number"], .media-frame input[type="search"], .media-frame input[type="email"], .media-frame input[type="url"], .media-frame textarea, .media-frame select { font-family: "Open Sans", sans-serif !important; }';

@george J I added a few changes more in your plugin to add support for admin bar in front side (not only in wp-admin), please correct me if some is wrong:

if (!defined('ABSPATH'))
	exit;

require_once( ABSPATH . "wp-includes/pluggable.php" );

if (is_user_logged_in()) {

	class OpenSans_FTW {

		protected static $instance = null;
		public $plugin_slug = 'opensans-ftw';

		public static function get_instance() {
			if (null == self::$instance) {
				self::$instance = new self;
			}

			return self::$instance;
}

		public function __construct() {
			$wp_version = get_bloginfo('version');
			if (version_compare($wp_version, '4.6', '<')) {
				return;
			}

			if(is_admin()) {
				add_action('admin_enqueue_scripts', array($this, 'add_assets'));
			} else {
				add_action('wp_footer', array($this, 'add_assets'));
			}
		}

		public function add_assets() {
			// add font
			wp_enqueue_style($this->plugin_slug, '//fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,300,400,600&subset=latin,latin-ext');

			// add custom css
			if(is_admin()) {
				$custom_css = 'body, #wpadminbar *:not([class="ab-icon"]), .wp-core-ui { font-family: "Open Sans", sans-serif !important; }';

				wp_add_inline_style($this->plugin_slug, $custom_css);
			} else {
				$custom_css = '#wpadminbar *:not([class="ab-icon"]) { font-family: "Open Sans", sans-serif !important; }';

				echo "<style type='text/css' media='all'>\n";
				echo "$custom_css";
				echo "</style>\n";
			}
		}
	}

	add_action('plugins_loaded', array('OpenSans_FTW', 'get_instance'));
}

@darko A7 Thanks a lot.