Detect if OS is in dark mode with Vanilla JS and add class to BODY

dark-mode.js

document.addEventListener("DOMContentLoaded", function(){
    // Detect dark mode with Vanilla JS and add class to BODY
    if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) { 
        document.body.classList.add("dark-mode"); // your class here
    } 
    
    // Turn Dark Mode on/off, manually
    document.querySelector('.lightsoff').onclick = function() {
        document.querySelector('body').classList.toggle('dark-mode');
    }
});