ToolbarMenuLinkTree.php

<?php

namespace Drupal\toolbar\Menu;

use Drupal\Core\Menu\MenuLinkTree;

/**
 * Extends MenuLinkTree to add specific theme suggestions for the toolbar.
 */
class ToolbarMenuLinkTree extends MenuLinkTree {
    
    /**
     * {@inheritdoc}
     */
    public function build(array $tree, $level = 0) {
        if ($level == 0) {
            if (!$tree) {
                return [];
            }
            $build = parent::build($tree, $level);
            
            /** @var \Drupal\Core\Menu\MenuLinkInterface $link */
            $first_link = reset($tree)->link;
            // Get the menu name of the first link.
            $menu_name = $first_link->getMenuName();
            // Add a more specific theme suggestion to differentiate this rendered
            // menu from others.
            $build['#menu_name'] = $menu_name;
            $build['#theme'] = 'menu__toolbar__' . strtr($menu_name, '-', '_');
            return $build;
        }
        else {
            return parent::build($tree, $level);
        }
    }

}