Adding external/inline resources via #attached in a Drupal 8 theme

stark2.theme.php

<?php
/**
 * @file
 * Example of how to attach external and inline resources for a theme via the
 * #attached render array method in 8.x.
 */

/**
 * Helper function for attaching resources to a render array.
 */
function _stark2_attach_resources(&$element) {
  $element['#attached']['css'][] = array(
    'type' => 'external',
    'data' => '//cdn.jsdelivr.net/alertify.js/0.4.0rc1/themes/alertify.css',
    'every_page' => TRUE,
  );
  $element['#attached']['css'][] = array(
    'type' => 'external',
    'data' => '//cdn.jsdelivr.net/alertify.js/0.4.0rc1/themes/alertify.default.css',
    'every_page' => TRUE,
  );
  $element['#attached']['js'][] = array(
    'type' => 'external',
    'data' => '//cdn.jsdelivr.net/alertify.js/0.4.0rc1/alertify.js',
    'every_page' => TRUE,
  );
  $element['#attached']['js'][] = array(
    'type' => 'inline',
    'data' => '(function($){ $(document).ready(function () { Alertify.dialog.alert("This is the Alertify.js plugin. Drupal loaded external CSS and JS resources and then this inline script to display this message."); }); })(jQuery)',
    'every_page' => TRUE,
  );
}

/**
 * Implements hook_page_alter().
 */
function stark2_page_alter(&$page) {
  // Works.
  _stark2_attach_resources($page);
}

/**
 * Implements hook_preprocess_THEME_HOOK().
 */
function stark2_preprocess_page(&$variables) {
  // Does not work. The main argument passed, $variables, is not a
  // render array. It contains the variables sent to the page template.
  // _stark2_attach_resources($variables);

  // Does not work. I am assuming due to some early aggregation/caching of the
  // page. Possible "bug", but very likely this is also "works as designed".
  // _stark2_attach_resources($variables['page']);

  // Works, however using hook_preprocess_page() to do this is not recommended.
  // Use the above hook_page_alter() example instead.
  // _stark2_attach_resources($variables['page']['content']);
}

/**
 * Implements hook_page_build().
 */
function stark2_page_build(&$page) {
  // Does not work. This is a module hook and will not be invoked in a theme.
  // _stark2_attach_resources($page);
}

Keep in mind the helper function _stark2_attach_resources() I created is just so I didn't have to duplicate the code for each function and I could just comment in/out which hook I wanted to test. In reality, you'd just put the attachments directly in hook_page_alter() and use $page['#attached'].

In case of need, you should look at hook_page_attachments_alter.

/**
 *  Implements hook_page_attachments_alter().
 */
function <your module>_page_attachments_alter(array &$page)
{
    $page['#attached']['css'][] = array(
        'type' => 'external',
        'data' => '//fonts.googleapis.com/css?family=Open+Sans',
        'every_page' => TRUE,
    );
}