How to call a function on PayUMoney Bolt Launch?

Question

We are using PayUMoney Bolt into website.

We have integrated PHP Kit provided by PayUMoney (Screenshot added). I need to add CSS on Bolt Launch.

Bolt popup loads into iframe so I am not able to write CSS for iframe content. I have to add CSS using javascript after Bolt popup load.

bolt.launch({
       key: $('#pkey').val(),
       txnid: $('#txnid').val(), 
       hash: h,
       amount: $('#payu_amt').val(),
       firstname: $('#firstname').val(),
       email: $('#payu_email').val(),
       phone: parseInt(''),
       productinfo: $('#productinfo').val(),
       surl : $('#surl').val(),
       furl: $('#furl').val(),
       mode: 'dropout' 
   },{ responseHandler: function(BOLT){
       alert( BOLT.response.txnStatus );     
   },
       catchException: function(BOLT){
           alert(BOLT.message );
       }
   });

Payumoney PHP script