How to insert CSS style into a frame that is being rendered by another script?

Question

css

Hello comrades! Need a crutch to help solve the problem.
The web page has a script that renders a frame from another site. You need to embed in this frame and include your CSS file there.

How can this be done?

Answer 1

Try this, just specify to whom you assign the file connection

document.addEventListener("DOMContentLoaded", () => {
let cssLink = document.createElement("link");
cssLink.href = "путь к файлу css";
cssLink.rel = "stylesheet";
cssLink.type = "text/css";
setTimeout(() => {
document.querySelector('iframe#form').contentDocument.head.appendChild(cssLink);
}, 2000);
});