Load css classes after using ajax calls

Styles won’t be affected if you use different selectors. Create a custom stylesheet, and enqueue it immediatelly. When the page is ready just apply the styles to the mentioned responsive div, or create some logic behind it.

So for example if you have
<div class="custom-sidebar">Some content</div>

Enqueue custom style within functions.php

wp_enqueue_style( 'some-custom-styles', get_stylesheet_uri() . "/custom.css" );

Afterwards just assign whichever class you want

$( ".custom-sidebar" ).addClass( ".myClassFromCustomCSS" );