PostCSS Light Dark Function

npm install @csstools/postcss-light-dark-function --save-dev

PostCSS Light Dark Function lets you use the light-dark color function in CSS, following the CSS Color 5 Specification.

Read more about this feature on mdn:

• define the colors for light and dark with light-dark()
• define which elements support light and/or dark with color-scheme

With both features combined you can mix and match color schemes in a single document, while also respecting the user's preferences.

.foo {
	color: light-dark(pink, magenta);
}

.bar {
	--bar: light-dark(cyan, deepskyblue);
}

/* becomes */

.foo {
	--csstools-light-dark-toggle--0: var(--csstools-color-scheme--light) magenta;
	color: var(--csstools-light-dark-toggle--0, pink);
	color: light-dark(pink, magenta);
}

.bar {
	--csstools-light-dark-toggle--1: var(--csstools-color-scheme--light) deepskyblue;
	--bar: var(--csstools-light-dark-toggle--1, cyan);
	@supports not (color: light-dark(tan, tan)) {

		& * {
	--csstools-light-dark-toggle--1: var(--csstools-color-scheme--light) deepskyblue;
	--bar: var(--csstools-light-dark-toggle--1, cyan);
		}
	}
}

@supports (color: light-dark(red, red)) {
.bar {
	--bar: light-dark(cyan, deepskyblue);
}
}

Declare that your document supports both light and dark mode:

:root {
	color-scheme: light dark;
}

/* becomes */

:root {
	--csstools-color-scheme--light: initial;
	color-scheme: light dark;
}@media (prefers-color-scheme: dark) {:root {
	--csstools-color-scheme--light:  ;
}
}

Dynamically alter the supported color scheme for some elements:

:root {
	/* Root only supports light mode */
	color-scheme: light;
}

.foo {
	/* This element and its children only support dark mode */
	color-scheme: dark;
}

/* becomes */

:root {
	/* Root only supports light mode */
	--csstools-color-scheme--light: initial;
	color-scheme: light;
}

.foo {
	/* This element and its children only support dark mode */
	--csstools-color-scheme--light:  ;
	color-scheme: dark;
}

Usage

Add PostCSS Light Dark Function to your project:

npm install postcss @csstools/postcss-light-dark-function --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssLightDarkFunction = require('@csstools/postcss-light-dark-function');

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

PostCSS Light Dark Function runs in all Node environments, with special instructions for:

• Node
• PostCSS CLI
• PostCSS Load Config
• Webpack
• Next.js
• Gulp
• Grunt

Options

preserve

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

postcssLightDarkFunction({ preserve: false })

.foo {
	color: light-dark(pink, magenta);
}

.bar {
	--bar: light-dark(cyan, deepskyblue);
}

/* becomes */

.foo {
	--csstools-light-dark-toggle--0: var(--csstools-color-scheme--light) magenta;
	color: var(--csstools-light-dark-toggle--0, pink);
}

.bar {
	--csstools-light-dark-toggle--1: var(--csstools-color-scheme--light) deepskyblue;
	--bar: var(--csstools-light-dark-toggle--1, cyan);
	& * {
	--csstools-light-dark-toggle--1: var(--csstools-color-scheme--light) deepskyblue;
	--bar: var(--csstools-light-dark-toggle--1, cyan);
	}
}

enableProgressiveCustomProperties

The enableProgressiveCustomProperties option determines whether the original notation is wrapped with @supports when used in Custom Properties. By default, it is enabled.

Note

We only recommend disabling this when you set preserve to false or if you bring your own fix for Custom Properties.
See what the plugin does in its README.

postcssLightDarkFunction({ enableProgressiveCustomProperties: false })