css-prefers-color-scheme

Prefers Color Scheme

npm install css-prefers-color-scheme --save-dev

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

To use this feature you need to do two things :

• add the PostCSS plugin that transforms the media queries
• add the browser polyfill that triggers specific queries in a browser

@media (prefers-color-scheme: dark) {
	:root {
		--site-bgcolor: #1b1b1b;
		--site-color: #fff;
	}
}

@media (prefers-color-scheme: light) {
	:root {
		--site-bgcolor: #fff;
		--site-color: #222;
	}
}

/* becomes */

@media (color: 48842621) {
	:root {
		--site-bgcolor: #1b1b1b;
		--site-color: #fff;
	}
}

@media (prefers-color-scheme: dark) {
	:root {
		--site-bgcolor: #1b1b1b;
		--site-color: #fff;
	}
}

@media (color: 70318723) {
	:root {
		--site-bgcolor: #fff;
		--site-color: #222;
	}
}

@media (prefers-color-scheme: light) {
	:root {
		--site-bgcolor: #fff;
		--site-color: #222;
	}
}

Usage

Add Prefers Color Scheme to your project:

npm install postcss css-prefers-color-scheme --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const prefersColorScheme = require('css-prefers-color-scheme');

postcss([
	prefersColorScheme(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

Options

preserve

The preserve option determines whether the original notation is preserved. By default, it is preserved.

prefersColorScheme({ preserve: false })

@media (prefers-color-scheme: dark) {
	:root {
		--site-bgcolor: #1b1b1b;
		--site-color: #fff;
	}
}

@media (prefers-color-scheme: light) {
	:root {
		--site-bgcolor: #fff;
		--site-color: #222;
	}
}

/* becomes */

@media (color: 48842621) {
	:root {
		--site-bgcolor: #1b1b1b;
		--site-color: #fff;
	}
}

@media (color: 70318723) {
	:root {
		--site-bgcolor: #fff;
		--site-color: #222;
	}
}

Browser

// initialize prefersColorScheme (applies the current OS color scheme, if available)
import prefersColorSchemeInit from 'css-prefers-color-scheme/browser';
const prefersColorScheme = prefersColorSchemeInit();

// apply "dark" queries (you can also apply "light")
prefersColorScheme.scheme = 'dark';

or

<!-- When using a CDN url you will have to manually update the version number -->
<script src="https://unpkg.com/css-prefers-color-scheme@10.0.0/dist/browser-global.js"></script>
<script>prefersColorSchemeInit()</script>

[!TIP] Please use a versioned url, like this : https://unpkg.com/css-prefers-color-scheme@10.0.0/dist/browser-global.js Without the version, you might unexpectedly get a new major version of the library with breaking changes.

Prefers Color Scheme works in all major browsers, including Safari 6+ and Internet Explorer 9+ without any additional polyfills.

To maintain compatibility with browsers supporting prefers-color-scheme, the library will remove prefers-color-scheme media queries in favor of cross-browser compatible color media queries. This ensures a seamless experience, even when JavaScript is unable to run.

Browser Usage

Use Prefers Color Scheme to activate your prefers-color-scheme queries:

import prefersColorSchemeInit from 'css-prefers-color-scheme/browser';
const prefersColorScheme = prefersColorSchemeInit();

By default, the current OS color scheme is applied if your browser supports it. Otherwise, the light color scheme is applied. You may override this by passing in a color scheme.

import prefersColorSchemeInit from 'css-prefers-color-scheme/browser';
const prefersColorScheme = prefersColorSchemeInit('dark');

The prefersColorScheme object returns the following properties — scheme, hasNativeSupport, onChange, and removeListener.

scheme

The scheme property returns the currently preferred color scheme, and it can be changed.

import prefersColorSchemeInit from 'css-prefers-color-scheme/browser';
const prefersColorScheme = prefersColorSchemeInit();

// log the preferred color scheme
console.log(prefersColorScheme.scheme);

// apply "dark" queries
prefersColorScheme.scheme = 'dark';

hasNativeSupport

The hasNativeSupport boolean represents whether prefers-color-scheme is supported by the browser.

onChange

The optional onChange function is run when the preferred color scheme is changed, either from the OS or manually.

removeListener

The removeListener function removes the native prefers-color-scheme listener, which may or may not be applied, depending on your browser support. This is provided to give you complete control over plugin cleanup.

debug

If styles are not applied you can enable debug mode to log exceptions.

import prefersColorSchemeInit from 'css-prefers-color-scheme/browser';
const prefersColorScheme = prefersColorSchemeInit('light', { debug: true });

<script src="https://unpkg.com/css-prefers-color-scheme@10.0.0/dist/browser-global.js"></script>
<script>prefersColorSchemeInit('light', { debug: true })</script>

Browser Dependencies

Web API's:

• Window.matchMedia

ECMA Script:

• Object.defineProperty
• Array.prototype.forEach
• Array.prototype.indexOf
• RegExp.prototype.exec
• String.prototype.match
• String.prototype.replace

CORS

[!IMPORTANT] Applies to you if you load CSS from a different domain than the page.

In this case the CSS is treated as untrusted and will not be made available to the JavaScript polyfill. The polyfill will not work without applying the correct configuration for CORS.

Example :

page	css	CORS applies
https://example.com/	https://example.com/style.css	no
https://example.com/	https://other.com/style.css	yes

You might see one of these error messages :

Chrome :

DOMException: Failed to read the 'cssRules' property from 'CSSStyleSheet': Cannot access rules

Safari :

SecurityError: Not allowed to access cross-origin stylesheet

Firefox :

DOMException: CSSStyleSheet.cssRules getter: Not allowed to access cross-origin stylesheet

To resolve CORS errors you need to take two steps :

• add an HTTP header Access-Control-Allow-Origin: <your-value> when serving your CSS file.
• add crossorigin="anonymous" to the <link rel="stylesheet"> tag for your CSS file.

In a node server setting the HTTP header might look like this :

// http://localhost:8080 is the domain of your page!
res.setHeader('Access-Control-Allow-Origin', 'https://example.com');

You can also configure a wildcard but please be aware that this might be a security risk. It is better to only set the header for the domain you want to allow and only on the responses you want to allow.

HTML might look like this :

<link rel="stylesheet" href="https://example.com/styles.css" crossorigin="anonymous">

Using with Next.js

Given that Next.js imports packages both on the browser and on the server, you need to make sure that the package is only imported on the browser.

As outlined in the Next.js documentation, you need to load the package with a dynamic import:

useEffect(async () => {
	const prefersColorSchemeInit = (await import('css-prefers-color-scheme/browser')).default;
	const prefersColorScheme = prefersColorSchemeInit();
}, []);

How does it work?

Prefers Color Scheme is a PostCSS plugin that transforms prefers-color-scheme queries into color queries. This changes prefers-color-scheme: dark into (color: 48842621) and prefers-color-scheme: light into (color: 70318723).

The frontend receives these color queries, which are understood in all major browsers going back to Internet Explorer 9. However, since browsers can only have a reasonably small number of bits per color, our color scheme values are ignored.

Prefers Color Scheme uses a browser script to change (color: 48842621) queries into (max-color: 48842621) in order to activate “dark mode” specific CSS, and it changes (color: 70318723) queries into (max-color: 48842621) to activate “light mode” specific CSS.

@media (color: 70318723) { /* 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. These are suffixed with a random large number.