Use jss and postcss together in your project.

jss-postcss-precompile-example.js

// If you are planning to use postcss plugins like autoprefixer, 
//you probably should precompile your jss style objects with postcss-js before running your app.
// Because autoprefixer makes a network request and loads caniusedb json file into your bundle 
//and this will dramaticly increase the size of your bundle.

// So lets create a precompile script

// Following 2 library needed for writing compiled jss style objects to files
var fs = require("fs");
var stringifyObject = require("stringify-object"); 
// Create postcss parser object
var postcssJs = require("postcss-js");
var autoprefixer = require("autoprefixer");
var postcss = postcssJs.sync([ autoprefixer ]);

// A sample jss style objects that will be compiled by postcss
const homeStyles = {
  header: {
    'display': 'flex',
    'flex-direction': 'column',
    'align-items': 'center',
    'margin-top': '8rem'
  },
  headerDark: {
    extend: 'header',
    'color': '#000'
  },
  headerLight: {
    extend: 'header',
    'color': '#fff'
  },
  title: {
    'text-align': 'center',
    'padding': '0 4rem',
    'margin-bottom': '2rem'
  }
}
const formStyles = {
  form: {
    'width': '80%',
    'display': 'flex',
    'flex-direction': 'column',
    'align-items': 'center'
  },
  label: {
    'display': 'block',
    'width': '100%',
    'font-size': '2rem',
    'margin-bottom': '3rem',
    'text-align': 'center'
  },
  input: {
    'font-size': '2rem',
    'padding': '1rem 2rem',
    'width': '100%',
    'margin-bottom': '3rem'
  }
}

// Create a jss container style object
// We want output our compiled jss style objects to files as "home.js" and "form.js"
// So we can use "import styles from '../styles/home.js'" in our components
const jssContainer = {
  home: homeStyles,
  form: formStyles
}

// This function compiles each class definition in every style object and 
//creates files according to the jss container
function saveJssChunksToFile( jssContainer ) {

  if(typeof jssContainer !== 'object') {
    return
  }

  for(var chunk in jssContainer) {
    // postcssJss is a custom function that compiles each jss class with postcss
    var data = 'export default '+stringifyObject( postcssJss( jssContainer[chunk] ) );
    // Write chunk to a file
    fs.writeFile('styles/'+chunk+'.js', data, function(err) {
      if(err) throw err;
    })
  }

  return
}

// This function compiles single jss style object with postcss
function postcssJss( jssStyleObject ) {

  var result = {};

  if(typeof jssStyleObject !== 'object') {
    return result;
  }

  // process every css class in the object
  for (var jssClass in jssStyleObject) {
    result[jssClass] = postcss(jssStyleObject[jssClass]);
  }

  return result;
}

// run
saveJssChunksToFile( jssContainer );

// adding npm script to the package.json recommended
// "scripts": {
//  ...
//  "styler": "node ./scripts/dev/createStyles.js"
//  ...
// },

jss-postcss-simple-example.js

// use postcss-js package to parse css-in-js styles before jss compiles it
import postcssJs from 'postcss-js'
// import postcss plugins you want to use in your project
import autoprefixer from 'autoprefixer'

// import jss and jss camelcase plugin. We must use camelcase plugin to let jss properly process postcss style objects
import { create } from 'jss'
import camelCase from 'jss-camel-case'

// create jss object
let jss = create()
jss.use(camelCase())

// create postcss parser with plugins we want to use
const postcss = postcssJs.sync([ autoprefixer ])

// a sample style object that gives jss the power of postcss
const testStyle = {
  testClass: postcss({
    transform: 'translateX(-50%) translateY(-50%)',
    filter: 'brightness(0)',
    display: 'flex',
    animation: 'explode 0.3s cubic-bezier(.12,.61,.26,.93) 0s 1 normal forwards'
  }),
  '@keyframes explode': {
    from: postcss({
      'filter': 'brightness(0)'
    }),
    to: postcss({
      'filter': 'brightness(1)'
    })
  },
}

In order to prevent this i wrote a small script that process all jss files with postcss before app bundling starts.

Agreed! I am doing the same, I build the files with a custom preprocessor based on postcss and then feed them to the $tyle function :)