Javascript cannot set innerHTML of button with id because it's "null"

My goal is to have a button that onclick will call a function to change the styling of the html tag and the text (or innerHTML) of the button itself. Shouldn't be that hard, right? Well...

HTML:

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>New look for my site!</title>
  <link rel="stylesheet" type="text/css" href="main.css">
  <script src="change-template.js"></script>
</head>
<body>
  <aside>
    <button type="button" id="template-button" onclick="changeTemplate()">Nightmode: On</button>
  </aside>
  <main>
    <h1>New page look for my site</h1>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
  </main>
</body>
</html>

CSS (main.css):

html{
  background: "#111";
  color: "#0F0";
}

JS (change-template.js):

var html = document.getElementsByTagName('html')[0];
var button = document.getElementById('template-button');
var nightmode = true;

function changeTemplate(){
  //change to lighter color if black and vice versa
  if(nightmode){
    html.style.background = "#EEE";
    html.style.color = "#000";
    button.innerHTML = "Nightmode: OFF";
  }else{
    html.style.background = "#111";
    html.style.color = "#0F0";
    button.innerHTML = "Nightmode: ON";
  }
  nightmode = !nightmode;
}

And I'm getting this error

Uncaught TypeError: Cannot set property 'innerHTML' of null at changeTemplate (change-template.js:10) at HTMLButtonElement.onclick (new-look.html:11)

So please help me it's 90 degrees and I'm sitting here on the verge of hunger stressed about this simple issue and can barely think.