/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_system_modules_alter(&$form) {
  $path = drupal_get_path('module', 'mymodule') . '/';
  $form['#attached']['js'][]  = $path . 'mymodule.js';
  $form['#attached']['css'][] = $path . 'mymodule.css';
}

Drupal 7.x