Prefers Color Scheme

Prefers Color Scheme lets you use light and dark color schemes in all
browsers, following the Media Queries specification.

Usage

From the command line, transform CSS files that use prefers-color-scheme
media queries:

npx css-prefers-color-scheme SOURCE.css TRANSFORMED.css

Next, use that transformed CSS with this script:

<link rel="stylesheet" href="TRANSFORMED.css">
<script src="https://unpkg.com/css-prefers-color-scheme/browser.min"></script>
<script>
colorScheme = initPrefersColorScheme('dark') // apply "dark" queries (you can change it afterward, too)
</script>

Dependencies got you down? Don’t worry, this script is only 537 bytes.

Usage

• First, transform prefers-color-scheme queries using this
  PostCSS plugin.
• Next, apply light and dark color schemes everywhere using this
  browser script.

How does it work?

Prefers Color Scheme uses a PostCSS plugin to transform
prefers-color-scheme queries into color-index queries. This changes
prefers-color-scheme: dark into (color-index: 48),
prefers-color-scheme: light into (color-index: 70), and
prefers-color-scheme: no-preference into (color-index: 22).

The frontend receives these color-index queries, which are understood in all
major browsers going back to Internet Explorer 9. However, since browsers only
apply color-index queries of 0, our color scheme values are ignored.

Prefers Color Scheme uses a browser script to change
(color-index: 48) queries into not all and (color-index: 48) in order to
activate “dark mode” specific CSS, and it changes (color-index: 70) queries
into not all and (color-index: 48) to activate “light mode” specific CSS.

@media (color-index: 70) { /* prefers-color-scheme: light */
  body {
    background-color: white;
    color: black;
  }
}

Since these media queries are accessible to document.styleSheet, no CSS
parsing is required.

Why does the fallback work this way?

The value of 48 is chosen for dark mode because it is the keycode for 0,
the hexidecimal value of black. Likewise, 70 is chosen for light mode because
it is the keycode for f, the hexidecimal value of white.