When AJAX mode is enabled in Subscribe2 the usual form is replaced by a jQuery UI mods box. jQuery UI has many CSS themes to choose from and this hook allows you to easily use another CSS style from a CDN.

function custom_ajax_css() {
    return 'http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/themes/ui-lightness/jquery-ui.css';
}
add_filter('s2_jqueryui_css', 'custom_ajax_css');