In order to style an iframe, you will need to have access to whatever is generating it, just like with any other web document.
An iframe is a different (child) document loading inside a designated area of your parent one.
Mainly as a precaution against cross-frame scripting, communication between the iframe
and the parent is disabled by default but can be enabled if the child document allows it. With this communication closed, responsiveness becomes a major issue.
Note it is possible for a page to check if it is rendered inside an iframe and forbid it (refuse to render) or display a nasty custom message informing the visitor it doesn't belong there.
Also, please note iframes should be avoided for accessibility reasons, as many handheld devices display them awkwardly and some don't display the iframe contents at all.