Magento 2 add parent category class to body for product listing page

Question

I have number of subcategories under a main category called 'services'. I want to add a custom class 'service-listing' to body on all the subcategory pages under this category.

Answer 1

I once did it with an observer.

First declare it in Namespace/ModuleName/etc/frontend/events.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="layout_load_before">
        <observer name="namespace_modulename_load_layout" instance="Namespace\ModuleName\Observer\LoadLayout" />
    </event>
</config>

Then create the file in app/code/Namespace/ModuleName/Observer/LoadLayout.php

namespace Namespace\ModuleName\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\View\Page\Config;

class LoadLayout implements ObserverInterface
{

    public function __construct(
        Config $pageConfig,
        \Magento\Framework\Registry $registry
    ) {
        $this->_pageConfig = $pageConfig;
        $this->_registry = $registry;
    }

    /**
     * @param Observer $observer
     * @return $this
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $parentCategoryId = $this->getLevel1Category();

        if($parentCategoryId=='your specific category id'){
            $this->_pageConfig->addBodyClass('your wanted body class');
        }

        return $this;
    }

    public function getLevel1Category(){
        if($this->getCurrentCategory()){
            if($this->getCurrentCategory()->getParentCategories()){
                foreach ($this->getCurrentCategory()->getParentCategories() as $parent) {
                    if ($parent->getLevel() == 2) { //the level could be different depending on your category architecture
                        return $parent->getId();
                    }
                }
            }
        }
        return null;
    }

    private function getCurrentCategory()
    {
        return $this->_registry->registry('current_category');
    }
}

The part to retrieve the category ID was based on this : Magento 2 How to get first level parent category of current Category?