Simply put, it's not possible.
The reason being:
In order to modify the style within an <iframe>
, you must utilize Javascript.
To achieve this, you will first need to obtain the DOM element
let redPlayButton = document.getElementById('youtube-iframe').contentWindow.document.getElementsByClassName('ytp-large-play-button')[0];
and proceed with making alterations to it:
redPlayButton.style.background = "https://something-here";
Nevertheless, due to the restrictions imposed by the Same-origin policy, such modifications are limited to an <iframe>
sourced from your own server.
Accessing a different origin <iframe>
using JavaScript is prohibited for security reasons. Rigorous enforcement of the same-origin policy ensures that scripts attempting to access a frame from an alternate origin are actively blocked by web browsers.