How to do use a different theme on search result page instead of default?

You can assign custom theme using Event/Observer for Search result page

Create events.xml at app/code/{VendorName}/{Modulename}/etc/frontend/ and call an observer on layout_load_before event.

<?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="ForcetoChangeTheme" 
            instance="{VendorName}{Modulename}ObserverChangeTheme" />
    </event>
</config>

On the observer ChangeTheme.php at {VendorName}{Modulename}Observer change the theme programmatically.

<?php
namespace DevamitberaChangethemeAtCheckoutObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppRequestDataPersistorInterface;
use MagentoFrameworkAppObjectManager;

class ChangeTheme implements ObserverInterface
{
     /**
     * Design package instance
     *
     * @var MagentoFrameworkViewDesignInterface
     */
    private $_design;
    /**
     * Store manager
     *
     * @var MagentoStoreModelStoreManagerInterface
     */
    private $_storeManager;
    private $request;
    private $logger;

    public function __construct(
        MagentoFrameworkViewDesignInterface $design,
        MagentoStoreModelStoreManagerInterface $storeManager,   
        MagentoFrameworkAppRequestHttp $request,
        PsrLogLoggerInterface $logger
    ) {
        $this->_design = $design;
        $this->_storeManager = $storeManager;
        $this->request = $request;
        $this->logger = $logger;
        }

    /**
     *
     * @param MagentoFrameworkEventObserver $observer
     * @return void
     */
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $controller = $observer->getControllerAction();
        //$this->logger->info($this->request->getFullActionName());
        if($this->request->getFullActionName() =='catalogsearch_result_index'){
            $this->_design->setDesignTheme(1); //  1 is Theme id
        }
    }

}