Cannot get postcss to work at all

I've tried to extract the css into a styles.css file, as well as without any options just trying to use the css.

```
import babel from 'rollup-plugin-babel'
import builtins from 'rollup-plugin-node-builtins';
import commonjs from 'rollup-plugin-commonjs'
import external from 'rollup-plugin-peer-deps-external'
import postcss from 'rollup-plugin-postcss'
import resolve from 'rollup-plugin-node-resolve'
import url from 'rollup-plugin-url'
import typescript from 'rollup-plugin-typescript'

import pkg from './package.json'

export default {
input: 'src/index.js',
output: [
{
file: pkg.main,
format: 'cjs',
},
{
file: pkg.module,
format: 'es',
}
],
plugins: [
builtins(),
typescript(),
external(),
postcss({
extract: {
path: 'styles.css'
},
plugins: []
}),
url(),
babel({
exclude: 'node_modules/**',
plugins: [ 'external-helpers' ]
}),
resolve(),
commonjs()
]
}
```