[1100] PostCSS plugins don't do anything

I'm trying to add some PostCSS plugins. Currently trying to get pxtorem working, but my compiled CSS is unchanged. Here's the relevant config:

module.exports = {
  css: [
    { src: '~assets/scss/main.scss', lang: 'scss' }
  ],

  build: {
    postcss: [
      require('postcss-pxtorem')({
        propList: ['*']
      })
    ]
  }
}