Instant dark mode (bookmarklet)

Instant-Dark-Mode.js

// Thanks to https://twitter.com/derekstavis/status/1306021971163926528 !
// Install this as a bookmarklet, use a tool like https://caiorss.github.io/bookmarklet-maker/

const styleId = 'Instant-Dark-Mode'
const THEME = 'Instant-Dark-Mode'
const THEME_DARK = `${THEME}--dark`
const THEME_LIGHT = `${THEME}--light`

let element = document.getElementById(styleId)
if (!element) {
  const s = document.createElement('style')
  s.id = styleId
  s.innerHTML = `body.${THEME}.${THEME_DARK} {
      background-color: black;
      filter: hue-rotate(180deg) invert(90%);
  }`
  document.body.appendChild(s)
}

const classList = document.body.classList
classList.add(THEME)
const wasDark = classList.contains(THEME_DARK)
const isDark = !wasDark
classList.toggle(THEME_DARK, isDark)
classList.toggle(THEME_LIGHT, !isDark)

To apply "permanently" to a website, with the help from some userstyles extension like Stylus, use this style:

body {
  background-color: black;
  filter: hue-rotate(180deg) invert(90%);
}