Modify the HTML of the opt-in iframe

Hello. I found this wonderful plugin to add a CSS to the opt-in iframe:

Problem is, it looks like there’s little to style, since I see a simple body tag that doesn’t accept too much styles (eg in HTML5).

Is there any way to modify the HTML, eg. adding a <p>?

I could not locate the file where the HTML is generated.

Any help is appreciated!