Snippet to add some js or css to the Drupal admin theme via a module.

custom_admin.module

<?php

// Adds to EVERY admin page
function custom_admin_page_build(&$page) {
  // Test if on an admin page
  if (path_is_admin(current_path())) {
    // Get paths
    $path = drupal_get_path('module', 'custom_admin');
    $theme_path = drupal_get_path('theme', 'themekit');
    // Add js and css to customize the admin theme
    $page['page_bottom']['custom_admin'] = array(
      '#attached' => array(
        'js' => array(
          // Select2
          $path . '/select2/dist/js/select2.min.js' => array('every_page' => true),
          // Select2 init
          $path . '/js/admin-multi-selects.js' => array('every_page' => true),
        ),
        'css' => array(
          // Select2 css
          $path . '/select2/dist/css/select2.min.css' => array('every_page' => true),
          // Custom css
          $path . '/css/select2-custom.css' => array('every_page' => true),
        ),
      )
    );

  }
}

// Add only when a specific element is present (in this case, selects)
function custom_admin_preprocess_form_element(&$variables) {
  $is_admin = &drupal_static(__FUNCTION__ . 'is_admin');
  $is_admin = (isset($is_admin)) ? $is_admin : path_is_admin(current_path());

  if ($is_admin && isset($variables['element'])) {
    $element =& $variables['element'];
    if ($element['#type'] == 'select') {
      // Get path to module
      $path = drupal_get_path('module', 'custom_admin');

      // Default options.
      $js_options = array('group' => JS_THEME);
      $css_options = array('group' => CSS_THEME);

      // Add js and css to customize the admin theme
      drupal_add_js($path . '/select2/dist/js/select2.min.js', $js_options);
      drupal_add_js($path . '/js/admin-multi-selects.js', $js_options);

      drupal_add_css($path . '/select2/dist/css/select2.min.css', $css_options);
      drupal_add_css($path . '/css/select2-custom.css', $css_options);
    }
  }
}