How can I add content to the page by just using css/js?

How can I add content to the page by just using css/js?

I'm developing a real estate website. The website is working with a paid
service that allows people to view real estate listings through my
website. My website becomes a subdomain of the other website like
www.mywebsite.listingswebsite.com. The listings website dynamically pulls
the header and footer wrappers on one of the pages I provided them, and
puts their content in the middle of the page. So it looks like this.
My Header
Their listings content that I have no control over
My Footer
The problem is, I want to modify their page. I want to add some content
before their content, I want to move elements around, and I want to delete
some elements. For now, all I know is how to remove elements from the page
- and that's by finding the ID of their Div, and adding display:none in my
CSS file.
But what if I wanted to add content to the page? I only have control over
the CSS/JS, so how would I do that? Say I wanted to add a picture right
before their content.