Create a switcher on navigation bar,

<button type="button" name="dark_light" onclick="toggleDarkLight()" title="Toggle dark/light mode">
  <i id="darkSwitcher" class="fas fa-moon"></i>
</button>

Create a js file and include it in the footer,

var switcher = document.getElementById('dark-switcher');
switcher.onclick = function(){
  bd = document.getElementsByTagName("BODY")[0]; // get body tag
  if (bd.classList.contains('dark')) { // current: dark -> want: bright
    switcher.children[0].className = 'fas fa-moon'; // chan
    bd.classList.remove('dark');
    localStorage.setItem('dark-status', 'bright'); // store the current preference of the user
  } else{ // current: bright -> want: dark
    switcher.children[0].className = 'fas fa-sun';
    bd.classList.add('dark');
    localStorage.setItem('dark-status', 'dark'); // store the current preference of the user
  }
}

Add below code inside <head>. This code will check the current preference of the user (If they already chosen dark mode at the last time they visited website, the site will automatically change to dark mode).

<script>
// run on loading the page
const checkDarkModeStatus = () => {
  darkStatus = localStorage.getItem('dark-status');
  if (darkStatus == 'dark'){ // the last time user left is on 'dark mode'
    document.getElementsByTagName("BODY")[0].classList.add('dark');
    document.getElementById('darkSwitcher').className = 'fas fa-sun';
  } else {
    document.getElementsByTagName("BODY")[0].classList.remove('dark');
    document.getElementById('darkSwitcher').className = 'fas fa-moon';;
  }
}
window.onload = checkDarkModeStatus;
</script>