npm install --save-dev postcss postcss-functions

import fs from 'fs';
import postcss from 'postcss';
import functions from 'postcss-functions';
const options = {
	//options
};
const css = fs.readFileSync('input.css', 'utf8');
postcss()
  .use(functions(options))
  .process(css)
  .then((result) => {
    const output = result.css;
  });

body {
  prop: foobar();
}

import postcssFunctions from 'postcss-functions';
import { fromString, fromRgb } from 'css-color-converter';

function darken(value, frac) {
  const darken = 1 - parseFloat(frac);
  const rgba = fromString(value).toRgbaArray();
  const r = rgba[0] * darken;
  const g = rgba[1] * darken;
  const b = rgba[2] * darken;
  return fromRgb([r,g,b]).toHexString();
}

postcssFunctions({
  functions: { darken }
});

.foo {
  /* make 10% darker */
  color: darken(blue, 0.1);
}