Drupal 9: How to check if current page is a node and add a node bundle css class to body

In your custom theme theme.inc file for example add this code snippet (replace custom_theme with your custom theme name).

use Drupal\node\NodeInterface;

/**
 * @file
 * Custom theme hooks.
 */


function custom_theme_preprocess_html(&$variables) {

  $route = \Drupal::routeMatch();

  $node = $route->getParameter('node');
  if ($node instanceof NodeInterface) {
    $variables['attributes']['class'][] = 'page--node__' . $node->bundle();
  }
}

First we get the $route object and then we try to load a node object (if this is in fact a node page and the node object is an instance of a NodeInterface Class). We want check the node is a NodeInterFace class because there are routes where {node} is used but it is not upcasted, for example if the controller does not type hint it as NodeInterface.

So after we make sure that this is a node object then we get the bundle of the node add it as a css class in the body element of the page. The result for a node article page will be something like class="page--node__article".