I’ve to add custom CSS for particular store on checkout page, so I’ve updated a layout using observer.

Please follow the below method. you need to copy the below event in your module.

<?xml version="1.0" encoding="UTF-8" ?>
<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="magemonkeys_customshipping_addcustomcheckouthandle" instance="MagemonkeysCustomshippingObserverAddCheckoutLayoutUpdateHandleObserver" />
    </event>
</config>

And you can copy the below code for add the layout handle in Observer class.

<?php

namespace MagemonkeysCustomshippingObserver;

use MagentoCatalogModelCategory as CategoryModel;
use MagentoFrameworkEvent;
use MagentoFrameworkEventObserver as EventObserver;
use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkRegistry;
use MagentoFrameworkViewLayout as Layout;
use MagentoFrameworkViewLayoutProcessorInterface as LayoutProcessor;

/**
 *  AddCheckoutLayoutUpdateHandleObserver
 */
class AddCheckoutLayoutUpdateHandleObserver implements ObserverInterface
{
   
    const LAYOUT_HANDLE_NAME = 'checkout_index_index_emp';
    const LAYOUT_HANDLE_NAME_CUSTOMER = 'customer_account_emp';

    public $storeManager;
	public $customerSession;

    public function __construct(
        MagentoStoreModelStoreManagerInterface $storeManager,
		MagentoCustomerModelSession $customer
    ) {
        $this->storeManager = $storeManager;        
        $this->customerSession = $customer;        
    }
	
	public function getStoreCode()
	{
		return $this->storeManager->getStore()->getCode();
	}

    /**
     * @param EventObserver $observer
     *
     * @return void
     */
    public function execute(EventObserver $observer)
    {
        /** @var Event $event */
        $event = $observer->getEvent();
        $actionName = $event->getData('full_action_name');
		
        if ($actionName === 'checkout_index_index') {
            /** @var Layout $layout */
            $layout = $event->getData('layout');

            /** @var LayoutProcessor $layoutUpdate */
            $layoutUpdate = $layout->getUpdate();

            // check if Category Display Mode is "Mixed"
            if($this->getStoreCode() == 'abc_store' && $this->customerSession->isLoggedIn())
			{
                $layoutUpdate->addHandle(static::LAYOUT_HANDLE_NAME);
            }
        }
    }
}

That’s it. Now you can use the checkout_index_index_emp.xml file in your view/frontend/layout directory and make the changes as per your requirement.