postcss-composer

A PostCSS plugin that provides composable utilities for scalable, theme-aware styling.
— Made for scalable UI design.
Includes color mode conversion, dynamic theming, mixins, and scale-based units — AST-aware & Tailwind-friendly.

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✨ Features

• ✅ Convert rem(), em(), and scale() units to responsive expressions
• 🎨 Parse rgb(), hsl(), lab(), oklch(), etc. with contextual var() resolution
• 🌗 Theme switching with themes() and @mixin dark, @mixin light, etc.
• 🧩 Mixins for hover, ltr/rtl, breakpoints, and more
• ⚙️ Fully configurable via postcss.config.js
• 🎯 Tailwind-compatible with support for arbitrary values and custom variants
• 🔍 AST-aware — resolves nested and inline variables properly

Required peer dependencies:

• postcss
• postcss-mixins
• postcss-nested
• postcss-values-parser

🚀 Installation

Install postcss-composer and required PostCSS plugins:

npm install postcss-composer postcss postcss-mixins postcss-nested postcss-values-parser -D

⚙️ Configuration

Add postcss-composer to your postcss.config.mjs:

export default {
  plugins: ['@tailwindcss/postcss', 'autoprefixer', 'postcss-composer']
};

With Option

export default {
  plugins: {
    '@tailwindcss/postcss': {},
    autoprefixer: {},
    'postcss-composer': {
      'themes-attr': 'class', // Change to match your theme attribute (e.g. 'data-theme', 'color-scheme')
      // Optional: register custom plugins or mixins
      plugins: {
        'postcss-import': {},
        'postcss-custom-media': { preserve: false }
      },
      // plugins: ['postcss-import', ['postcss-custom-media', { preserve: false }]] // Alternative syntax
      mixins: {
        // Register custom mixin
        customHover: {
          '@media (hover: hover)': {
            '&:hover': {
              '--color': '#f00',
              '@mixin-content': {} // Use the mixin content
            }
          }
        }
      }
    }
  }
};

Note:

• themes-attr: The attribute name for theme switching (e.g. class, data-theme, color-scheme), default value is 'class'.
• plugins: An array of PostCSS plugins to be used for theme switching. You can us the same syntax as in the postcss.config.mjs file.
• mixins: An object with custom mixin names as keys and mixin functions as values. These mixins will be available in your CSS files.

💡 Usage

Once configured, you can use dynamic functions like scale(), rem(), em(), and themes() in your CSS:

/* css */
:root {
  --muted: #151619;
  --foreground: #18191d;
  --emphasis: #171717;
}

.btn {
  color: var(--emphasis); /* #171717 */
  border-color: hls(var(--muted) / 0.5);
  /* Converts to: hsl(225deg 10% 9% / 0.5) — currently supports rgb(), hsl(), hwb(), oklch() */
  background-color: themes(rgb(26 27 30), var(--foreground));
  /* Resolves value based on theme context (e.g. dark/light/class) */
  font-size: rem(32); /* → calc(2rem * var(--scale, 1)) */
  letter-spacing: em(4); /* → calc(0.25em * var(--scale, 1)) */
}

🎨 Color conversion

Convert contextual CSS variables (even nested or fallback!) using any culori mode:

/* css */
:root {
  --brand: #1a1b1e;
}

.card {
  background: hsl(var(--brand) / 0.4); /* ✅ context-aware */
  border-color: oklch(var(--brand, #333) / 0.2); /* ✅ fallback supported */
  color: lab(var(--brand, #111, #fff) / 0.5); /* ✅ multi-level fallback */
}

Supported color formats (culori Mode):

'a98' | 'cubehelix' | 'dlab' | 'dlch' | 'hsi' | 'hsl' | 'hsv' | 'hwb' | 'itp' | 'jab' | 'jch' | 'lab' | 'lab65' | 'lch' | 'lch65' | 'lchuv' | 'lrgb' | 'luv' | 'okhsl' | 'okhsv' | 'oklab' | 'oklch' | 'p3' | 'prophoto' | 'rec2020' | 'rgb' | 'xyb' | 'xyz50' | 'xyz65' | 'yiq';

🧩 Mixins

postcss-composer includes helpful mixins to handle interaction state and layout direction.

• hover: Uses :hover or :active depending on device
• ltr: Styles for LTR mode only
• rtl: Styles for [dir="rtl"]
• light: Styles scoped to [themes-attr="light"]
• dark: Styles scoped to [themes-attr="dark"]

CSS variables

/* variables */

:root,
:host {
  @mixin light-root {
    --muted: #f0f0f0;
    --foreground: #202020;
    --emphasis: #000000;
    /* ...others */
  }
  @mixin dark-root {
    --muted: #1a1a1a;
    --foreground: #e0e0e0;
    --emphasis: #ffffff;
    /* ...others */
  }
}

CSS class

/* css */

.btn {
  font-size: rem(16);

  @mixin hover {
    &:not([data-loading]):not(:disabled):not([data-disabled]) {
      --muted: #cccccc;
      --foreground: #eeeeee;
      --emphasis: #ffffff;
    }
  }

  @mixin light {
    --muted: #f0f0f0;
    --foreground: #202020;
    --emphasis: #000000;
  }
  @mixin dark {
    --muted: #1a1a1a;
    --foreground: #e0e0e0;
    --emphasis: #ffffff;
  }

  @mixin ltr {
    margin-left: auto;
  }
  @mixin rtl {
    margin-right: auto;
  }

  @mixin max 768px {
    text-align: center;
  }
  @mixin min 768px {
    text-align: start;
  }
}

📐 Scale & Unit Helpers

scale(...) function can support formats like:

/* css */
.selector {
  font-size: rem(32px); /* → calc(2rem * var(--scale, 1)) */
  letter-spacing: em(4px); /* → calc(0.25em * var(--scale, 1)) */
  padding: scale(32px); /* → calc(2rem * 1) */
  gap: scale(24px, 3); /* → calc(1.5rem * 3) */
  width: scale(10rem, screen); /* → calc(10rem * var(--screen-scale, 1)) */
}

Examples scale function

Input CSS	Output CSS
scale(32)	calc(2rem * 1)
scale(32, 3)	calc(2rem * 3)
scale(32, large)	calc(2rem * var(--large-scale, 1))
scale(32, --large)	calc(2rem * var(--large-scale, 1))
scale(32, -large--scale)	calc(2rem * var(--large-scale, 1))
scale(32, large-screen)	calc(2rem * var(--large-screen-scale, 1))
scale(32, --large-screen)	calc(2rem * var(--large-screen-scale, 1))
scale(32, -large-screen--scale)	calc(2rem * var (--large-screen-scale, 1))

🌙 Theme Context

Resolve theme-aware values based on attribute context:

/* css */
.card {
  background: themes(#1a1b1e, var(--fg)); /* → themes(light, dark) */
  /* → resolves to --fg based on .dark or [data-theme="dark"] etc. */
}

Customizable via themes-attr config (e.g. class, data-theme, etc.)

🌀 TailwindCSS Compatibility

Works seamlessly inside Tailwind CSS with arbitrary value support:

// file.tsx
<span className="text-[themes(#1a1b1e,#fff)] [@mixin_ltr]:mr-auto" />

🧪 TailwindCSS v4

/* globals.css */
@import 'tailwindcss';

@custom-variant mixin-light (@mixin light);
@custom-variant mixin-dark (@mixin dark);
@custom-variant mixin-ltr (@mixin ltr);
@custom-variant mixin-rtl (@mixin rtl);

@utility color-* {
  color: themes(--value([ *]));
}
@utility background-* {
  background: themes(--value([ *]));
}

// file.tsx
<span className="color-[#1a1b1e,#fff] mixin-ltr:mr-auto" />

🛠️ Technical Notes

• 🎯 var() resolution:
  • Supports deeply nested values (e.g. var(--a, var(--b, #fff)))
  • Context-aware: resolves from closest rule → parent → root
• 🎨 Color handling:
  • Uses culori under the hood — all formats supported
  • Auto-formats based on target function (hsl(), rgb(), hwb(), oklch(), etc.)
• 🧩 @mixin support:
  • The @mixin directive is used to define a mixin, which is a reusable block of CSS code.
  • @mixin hover: will adjust between hover and active depending on the device (@media (hover: hover) vs none)
  • @mixin light & @mixin dark: take the themes-attr value you configured, so it's flexible and can be used with any theme.
• 📌 No caching to preserve dynamicity (e.g. :hover, @media, inline overrides)
  • The current approach avoids caching to allow for dynamic contexts like :hover, :active, @media, and inline styles ([--var:...]) that can produce different results depending on the DOM state. Caching is premature and can limit user experience and developer flexibility — the first thought was to keep the plugin AST-aware and context-sensitive. So yes, keep it uncached for now.

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🤝 Contributing

Want to help improve postcss-composer? Check out the contribution guide

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📄 License

MIT © Ilham Khoeri