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()
  ]
}