How to add custom css to url rewritten by worker

I’m new to workers and still learning. I have this code working, which simply masks the url:

addEventListener("fetch", event => {
  let requestURI = new URL(event.request.url);

  if (requestURI.hostname == "app.leadkeep.io") {
    event.respondWith(rewriteURL(event.request));
  }
  
})

async function rewriteURL(request) {
  var URI = new URL(request.url);

  URI.protocol = "https";
  URI.hostname = "app.activeflow.io";

  return fetch(URI, request);
}

This works well but on the page that gets loaded there is an image that I would like to hide or replace to complete the customization. Is it possible to add/inject some custom css that will hide or alter the image on the page?