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 */);

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 })