Add assets in theme preprocess functions

You can now really easily add new assets in template preprocess functions.

Drupal 7

function template_preprocess_maintenance_page(&$variables) {
  drupal_add_library('core', 'normalize');
  drupal_add_library('system', 'maintenance');
}

Drupal 8 <= beta1

function template_preprocess_maintenance_page(&$variables) {
  // @see system_page_build()
  $attached = array(
    '#attached' => array(
      'library' => array(
        'core/normalize',
        'system/maintenance',
      ),
    ),
  );
  drupal_render($attached);
}

Drupal 8 >=beta2

function template_preprocess_maintenance_page(&$variables) {
  $variables['#attached']['library'][] =  'core/normalize';
  $variables['#attached']['library'][] =  'system/maintenance';
}