Apply settings within Iframe

Hi there, I am looking for a plugin in which can allow me to make CSS alterations within an iframe. The iframe is created by a script, so within the Page it looks like:

<script src=”https://app.rockgympro.com/b/s/?aak=e1962a4e0fsakhafs731kjafshkjhfasf762fa”></script&gt;

This script then creates the iframe and embeds it. So I am trying to work out how to get my settings applied?

Thanks…