If you ever need to programmatically add a class to the body tag of your Magento stores, you’ll need to setup an event observer for the layout_load_before event. Within your observer you’ll then want to add Magento\Framework\View\Page\Config as a dependency, you’ll then use the addBodyClass method to add your class.

See below for a more detailed example. In my example, I’m adding the class customer-logged-in to the body when a customer is logged in.

Event: layout_load_before

File: 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="vendor_module_add_body_class" instance="Vendor\Module\Observer\AddClassToBody" />
    </event>
</config>

File: Observer/AddClassToBody.php

<?php
namespace Vendor\Module\Observer;

use Magento\Customer\Model\Session as CustomerSession;
use Magento\Framework\View\Page\Config as PageConfig;
use Magento\Framework\Event\ObserverInterface;

class AddClassToBody implements ObserverInterface
{
    /** @var PageConfig */
    protected $pageConfig;
      /** @var CustomerSession */
    protected $customerSession;

    public function __construct(PageConfig $pageConfig, CustomerSession $customerSession)
    {
        $this->pageConfig = $pageConfig;
        $this->customerSession = $customerSession;
    }

    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        if (!$this->customerSession->isLoggedIn()) {
            return;
        }

        $this->pageConfig->addBodyClass('customer-logged-in');
    }
}

Tip: Some class names in Magento are not very help when you’re using the use statement. I’ll often use an alias to refer to classes see below:

Example:

Directly using Magento\Customer\Model\Session in my constructor is long winded. But also, when using the use statement simply putting Session doesn’t mean too much either.

Instead I’ll do this: use Magento\Customer\Model\Session as CustomerSession;. Immediately it’s clear what the class is doing.

Another example: use Magento\Framework\View\Page\Config as PageConfig;. Config doesn’t mean anything useful, however PageConfig is much more meaningful as it provides that extra bit of context into what the class does.