How do I apply CSS formatting to the iframe macro in Confluence Cloud?

[Bryan Guffey]

Hello all-

I swear I should know this, but I guess I get to show my lack of understanding, and I can't find the answer anywhere: 

What is the format and structure I need to put into the iFrame macro configuration in order to apply CSS styling to the content inside? 

I've tried everything I can think of, and nothing is working, which tells me that I'm way off base. 

TIA 

[Jens Schumacher]

The content within an iframe is isolated from the parent page due to same-origin policy restrictions, which prevent external CSS from directly affecting the iframe’s content.

In other words, the CSS styles likely only apply to the iframe itself, not the content within the iframe.