how to make all websites dark mode code example

Example: how to implement dark mode in website

/* You could simply toggle a global class on the body */

// JavaScript
button.addEventListener('click', () => {
    document.body.classList.toggle('dark');
    localStorage.setItem('theme', document.body.classList.contains('dark') ? 'dark' : 'light');
});

if (localStorage.getItem('theme') === 'dark') {
    document.body.classList.add('dark');
}

// CSS
/* Light mode */
body {
  background: #fff;
  color: #000;
}

/* Dark mode */
body.dark {
  background: #000;
  color: #fff;
}