Can I add my styles to someone else's site?

Question

Google Chrome

I saw extensions in the chrom store, for example, for VK with the ability to replace the background with any picture, animated wallpaper or dark theme. So, does this work by replacing the style files, or does it change directly in the file when the page loads? The whole thing can be done by yourself limited to HTML, CSS, JS? I became interested in all this booze, scoured Google and didn’t really find any information, only creating an extension for Google itself.

Answer 1

var link = document.createElement("link");
link.rel = "stylesheet";
link.href = "YouHref";
document.head.appendChild(link);

specifying your YouHref the address of the location of the css file you will insert styles