Webpack: How to process CSS statically linked in index.ejs

I’ve just created a new CLI project with Webpack & Typescript.

I’ve manually added a minimal bootstrap CSS file to index.ejs in order to have some basic styles available during app strart up (JS loading, processing etc.):

    <link rel="stylesheet" href="assets/app-bootstrapper.css" />
  <body aurelia-app="main">

Also I’ve added some plugin to webpack.config.js to copy my assets folder to dist during build:

// ...
...when(production || server, new CopyWebpackPlugin([
      { from: 'src/assets', to: path.resolve(outDir, 'assets') }])),
// ...

1) Is this basically the correct way to go?
I’ve read this post but setting extractCss: true as mentioned there doesn’t seem to have any effect…

2) How can I also apply CSS processing to the bootstrap file?
All CSS in my app which is required by views is processed using PostCSS (right now only auto-prefixer) but my app-bootstrapper.css is not.
How can I also include app-bootstrapper.css in the processings?