[2889] sass source maps for independent sass files

My structuring could be wrong or I am missing some part of documentation that does this. Below I have shared the config. I believe the source maps are not produced. /assets/sass/my-bulma.sass

@import '~bulma/sass/utilities/initial-variables'

$green: #5fbc0a
$cyan: #5fd9eb
$info: $cyan

@import '~bulma/bulma'

/nuxt.config.js

module.exports = {
  /*
  ** Headers of the page
  */
  head: {
    title: 'xxx',
    meta: [
      { charset: 'utf-8' },
      { name: 'viewport', content: 'width=device-width, initial-scale=1' }
    ],
    link: [
      { rel: 'icon', type: 'image/x-icon', href: '/favicon.ico' }
    ]
  },
  css : [
    '@/assets/sass/my-bulma.sass'
  ],
  /*
  ** Build configuration
  */
  build: {
    postcss: {
      plugins: {
        'postcss-custom-properties': {
          warnings: false
        }
      }
    },
    cssSourceMap : true,
    /*
    ** Run ESLint on save
    */
    extend (config, { isDev, isClient }) {
      if (isDev && isClient) {
        config.module.rules.push({
          enforce: 'pre',
          test: /\.(js|vue)$/,
          loader: 'eslint-loader',
          exclude: /(node_modules)/
        })
      }
    }
  }
}

What is the actual way to structure the css in NUXT? with sourcemap