Rapyd custom css in Woocommerce

Hi all,

I am developing an online shop using Rapyd for payments, using the Rapyd plugin for Woocommerce. I’d like to add custom css to the checkout page. Within the #rapyd-checkout div, an iFrame loads an external html to make the payment. I’d like to edit the basic css, fonts, background and borders of this html. Is there any way to do this? Perhaps with javascript?

Thanks for any help.