PostCSS Custom Properties

 npm install postcss-custom-properties --save-dev

PostCSS Custom Properties lets you use Custom Properties in CSS, following
the CSS Custom Properties specification.

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid var(--border-color);
	color: var(--text-color);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

/* becomes */

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid rgb(0, 217, 255);
	border: 1px solid var(--border-color);
	color: rgb(255, 192, 211);
	color: var(--text-color);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

Note:

• Only processes variables that were defined in the :root or html selector.
• Locally defined variables will be used as fallbacks only within the same rule, but not elsewhere.
• Fallback values in var() will be used if the variable was not defined in the :root or html selector.

Usage

Add PostCSS Custom Properties to your project:

 npm install postcss postcss-custom-properties --save-dev

Use it as a PostCSS plugin:

 const postcss = require('postcss');
const postcssCustomProperties = require('postcss-custom-properties');

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

PostCSS Custom Properties 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 properties using
custom properties should be preserved in their original form. By default these are preserved.

Custom property declarations are always preserved only var() functions can be omitted.

 postcssCustomProperties({ preserve: false })

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid var(--border-color);
	color: var(--text-color);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

/* becomes */

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid var(--border-color);
	color: rgb(255, 192, 211);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

Modular CSS Processing

If you're using Modular CSS such as, CSS Modules, postcss-loader or vanilla-extract to name a few, you'll probably
notice that custom properties are not being resolved. This happens because each file is processed separately so
unless you import the custom properties definitions in each file, they won't be resolved.

To overcome this, we recommend using the PostCSS Global Data
plugin which allows you to pass a list of files that will be globally available. The plugin won't inject any extra code
in the output but will provide the context needed to resolve custom properties.

For it to run it needs to be placed before the PostCSS Custom Properties plugin.

 const postcss = require('postcss');
const postcssCustomProperties = require('postcss-custom-properties');
const postcssGlobalData = require('@csstools/postcss-global-data');

postcss([
	postcssGlobalData({
		files: [
			'path/to/your/custom-selectors.css'
		]
	}),
	postcssCustomProperties(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);