Service worker for injecting styles or scripts: https://dev.to/mbround18/how-to-hijack-your-head-38dn

foundry-service-worker.js

class ElementHandler {
  element(element) {
    element.append(`<link
      rel="stylesheet" 
      type="text/css" 
      data-id="foundry-login"
      href="https://cdn.jsdelivr.net/gh/TheEpicSnowWolf/Foundry-VTT-Prettier-Login-Screen@main/foundry_login.css"  
    >`, {html: true});
    console.log("injected");
  }
}

async function handleRequest(req) {
  const res = await fetch(req)
  
  if (res.url.includes('dnd.example.com/join')) {
    return new HTMLRewriter().on("head", new ElementHandler()).transform(res)
  }  
  
  return res;
}

addEventListener("fetch", (event) => {
  event.respondWith(
    handleRequest(event.request).catch(
      (err) => new Response(err.stack, { status: 500 })
    )
  );
});