'theme callback' and hook_custom_theme() replaced by theme negotiators

In Drupal 7, the active theme could be changed either with a 'theme callback' entry in hook_menu(), or with hook_custom_theme(). In Drupal 8, both methods have been replaced with theme negotiators.

Drupal 7

/**
 * Implements hook_menu()
 */
function example_menu() {
  $items['path'] = array(
    'page callback' => 'foo_bar',
    'theme callback' => 'example_theme_callback',
  );

  return $items;
}

/**
 * Always use stark on that menu route.
 */
function example_theme_callback() {
  return 'stark';
}

Drupal 8

For active themes by route you first have to define a service in your example.services.yml file:

services:
  theme.negotiator.example:
    class: Drupal\example\Theme\ThemeNegotiator
    tags:
      - { name: theme_negotiator, priority: 0 }

and then in "example/src/Theme/ThemeNegotiator.php" use that new defined service class:

namespace Drupal\example\Theme;

use Drupal\Core\Theme\ThemeNegotiatorInterface;
use Drupal\Core\Routing\RouteMatchInterface;

class ThemeNegotiator implements ThemeNegotiatorInterface {
  /**
   * {@inheritdoc}
   */
  public function applies(RouteMatchInterface $route_match) {
    // Use this theme on a certain route.
    // return $route_match->getRouteName() == 'example_route_name';

    // Or use this for more than one route:
    $possible_routes = array(
        'entity.taxonomy_term.add_form',
        'entity.taxonomy_term.edit_form'
    );

    return (in_array($route_match->getRouteName(), $possible_routes));
  }

  /**
   * {@inheritdoc}
   */
  public function determineActiveTheme(RouteMatchInterface $route_match) {
    // Here you return the actual theme name.
    return 'stark';
  }

}

For hook_custom_theme() you can use the same mechanism. If you want to take over all existing routes you have to choose a high priority.
In case you want to just change the default theme dynamically but let route specific theme negotiators still apply, use a priority < 0.