dark_mode.js

const dark = 'dark-mode'
const light = 'light-mode'
// put the opposite of the default value here, i.e. if you want the page in light theme by default, we put true
let dark_on = true

function toggleDarkLight () {
  dark_on = !dark_on
  const newClass = dark_on ? dark : light

  const all = document.getElementsByTagName('*')
  for (let i = 0, max = all.length; i < max; i++) {
    const classList = all[i].classList

    if (classList.contains(dark) || classList.contains(light)) {
      classList.remove(dark)
      classList.remove(light)
      classList.add(newClass)
    }

    if (classList.length == 0) {
      all[i].className = newClass
    }
  }
}

toggleDarkLight()