How to get this function to change style sheets

I am new to jS and trying some simple tasks but even these so-called simple tasks are proving difficult at times. Please help me understand why this doesn’t work. This snippet is to change style sheets by using a toggle. As you can probably work out.

I have two style sheets, a toggle (checkbox) and a function with an if else statement. Why can I not get this to work? Thank you

function toggleTheme() {

  let theme = document.getElementsByTagName('link')[0];

  if (theme.getAttribute('href') == 'darkstyle.css') {
    theme.setAttribute('href', 'lightstyle.css');
  } else {
    theme.setAttribute('href', 'darkstyle.css');
  }
}
<link rel="stylesheet" href="darkstyle.css" />

<link rel="stylesheet" href="lightstyle.css" />

<label class="switch">
  <input  type="checkbox" />
  <span onchange="toggleTheme()" class="slider round"></span>
</label>