Change apperance of login modal form module drupal 7 - drupal

Question

I installed the modal forms module in drupal 7 (https://drupal.org/project/modal_forms) to create a modal window login form. But now, I'm tryig to change its appereance with css but I don't know how to do it.
Anyone knows how to change for example the appereance of input submit button applying one css class?
This is my code:
function get_output_ajax(){
$output = array();
ctools_add_js('ajax-responder');
$output[] = ctools_modal_command_dismiss(t('Login success'));
if (isset($_GET['destination'])) {
$output[] = ctools_ajax_command_redirect($_GET['destination']);
}
elseif(module_exists('login_destination')) {
$destination = login_destination_get_destination('login');
$output[] = ctools_ajax_command_redirect($destination['path']);
}
else {
$output[] = ctools_ajax_command_reload();
}
return $output;
}
function modal_forms_login($js = NULL) {
// Fall back if $js is not set.
if (!$js) {
return drupal_get_form('user_login');
}
$output = get_output_ajax();
}
}
else{
$output = get_output_ajax();
}
print ajax_render($output);
} global $user;
if ($user->uid == 0) {
$output = ctools_modal_form_wrapper('user_login', $form_state);
if (!empty($form_state['executed'])) {
$output = get_output_ajax();
}
}
else{
$output = get_output_ajax();
}
print ajax_render($output);
}
Thanks in advance.

Answer 1

Here my solution, it works for me:
function modal_forms_login($js = NULL) {
// Fall back if $js is not set.
if (!$js) {
return drupal_get_form('user_login');
}
ctools_include('modal');
ctools_include('ajax');
$form_state = array(
// 'title' => t('Log in'),
'ajax' => TRUE,
);
global $user;
if ($user->uid == 0) {
$output = ctools_modal_form_wrapper('user_login', $form_state);
//Necesario para editar el boton del login
$cadena = $output[0]['output'];
$patrón = '/class="form-submit"/';
$sustitución = 'class="form-submit btn btn-primary"';
$output[0]['output'] = preg_replace($patrón, $sustitución, $cadena);
if (!empty($form_state['executed'])) {
$output = get_output_ajax();
}
}
else{
$output = get_output_ajax();
}
print ajax_render($output);
}