adaption css from a new on-page app in iframe off the main app

I need a professional with experience for CSS. The new application: [login to view URL] Is a one-page app without css. It runs in an iframe under the main app: [login to view URL] The design should be adapted to the existing page (Scrolled downwards): [login to view URL] Background: Customers can log in with their e-mail on the website, they can see their open bills and can pay them. The steps you can see in the file [login to view URL] I can supply the CSS files from the main application.