Source map for css file on classic theme

[30ml]

First I am not pro on js and webpack and node js and have following webpack.js

/**
 * 2007-2017 PrestaShop
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Academic Free License 3.0 (AFL-3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * https://opensource.org/licenses/AFL-3.0
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to [email protected] so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade PrestaShop to newer
 * versions in the future. If you wish to customize PrestaShop for your
 * needs please refer to http://www.prestashop.com for more information.
 *
 * @author    PrestaShop SA <[email protected]>
 * @copyright 2007-2017 PrestaShop SA
 * @license   https://opensource.org/licenses/AFL-3.0 Academic Free License 3.0 (AFL-3.0)
 * International Registered Trademark & Property of PrestaShop SA
 */
const webpack = require('webpack');
const path = require('path');
const ExtractTextPlugin = require("extract-text-webpack-plugin");

let config = {
  entry: {
    main: [
      './js/theme.js',
      './css/theme.scss'
    ]
  },
  output: {
    path: path.resolve(__dirname, '../assets/js'),
    filename: 'theme.js'
  },
  module: {
    rules: [
      {
        test: /\.js/,
        loader: 'babel-loader'
      },
      {
        test: /\.scss$/,
        use: ExtractTextPlugin.extract({
          fallback: 'style-loader',
          use: [
            {
              loader: 'css-loader',
              options: {
                minimize: true
              }
            },
            'postcss-loader',
            'sass-loader'
          ]
        })
      },
      {
        test: /.(png|woff(2)?|eot|ttf|svg)(\?[a-z0-9=\.]+)?$/,
        use: [
          {
            loader: 'file-loader',
            options: {
              name: '../css/[hash].[ext]'
            }
          }
        ]
      },
      {
        test : /\.css$/,
        use: ['style-loader', 'css-loader', 'postcss-loader']
      }
    ]
  },
  externals: {
    prestashop: 'prestashop',
    $: '$',
    jquery: 'jQuery'
  },
  plugins: [
    new ExtractTextPlugin(path.join('..', 'css', 'theme.css'))
  ]
};

config.plugins.push(
  new webpack.optimize.UglifyJsPlugin({
    sourceMap: false,
    compress: {
      sequences: true,
      conditionals: true,
      booleans: true,
      if_return: true,
      join_vars: true,
      drop_console: true
    },
    output: {
      comments: false
    },
    minimize: true
  })
);

module.exports = config;

Through this js while I do webpack-w then we can not see source map of my css. It simply output css only. I heard that we can check it's sourcemap but I do not know how to do it. That would be precious for me to develop theme. Your suggestion would be highly appreciate