Determine the active tab

>> I need to apply CSS or jQuery to the page when the last tab “Confirm Order” is active.

Can you please try the below code from your side?

add_action('wp_footer', 'thmscf_additional_css_specif_step', 999);
function thmscf_additional_css_specif_step() {
 if(is_checkout()){
   ?>
   <script type="text/javascript">
     (function($) {

     	$('#action-next').click(function(){
     		add_or_remove_additional_css();
     	});
     	$('#action-prev').click(function(){
     		add_or_remove_additional_css();
     	});

     	$('.thwmscf-tab').click(function(){
     		add_or_remove_additional_css();
     	});
     		
     	function add_or_remove_additional_css(){
     		if(($('#step-3')).hasClass('active')){
     			
     			// you can add CSS here by using js
     		}else{
     			
     			// you can revert CSS here by using js
     		}

     	}
     		    
     })(jQuery, window, document)
   </script>
   <?php
 }
}

We hope this helps.

Thank you!