How to inject a button to a webpage with Chrome Extension

I'm using content scripts and I would like to inject a button to a webpage.

This is my manifest.json:

{
  "manifest_version": 2,
  "name": "my extension",
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
      {
        "matches": ["http://127.0.0.1:8000/*"],
        "js": ["script.js"],
        "run_at": "document_end"
      }
  ]
}

This is popup.html:

<html>
  <body>
    <input type="button" id="button" style="display:none;">
  </body>
</html>

And script.js:

document.body.style.backgroundColor = "yellow";
var button = document.getElementById("button");
button.style.visibility = "visible";

When going to http://127.0.0.1:8000 I see the background changes to yellow, however it doesn't find button because it's not part of the webpage served by my local server. It shows this error:

Uncaught TypeError: Cannot read property 'style' of null

What should I do to inject a button on top of the content in http://127.0.0.1:8000 (assuming I don't know its content)?