Webpack config exports an object

extra-webpack.config.js

const { styles } = require('@ckeditor/ckeditor5-dev-utils');

module.exports = {
  module: {
    rules: [
      // Your existing rules (example)
      {
        test: /\.css$/,
        use: [
          {
            loader: 'style-loader',
            options: {
              // `style-loader` options here...
            }
          }
        ]
      },
      // CKEditor rules from the docs
      // https://ckeditor.com/docs/ckeditor5/latest/installation/getting-started/quick-start-other.html#building-the-editor-from-source
      {
        test: /ckeditor5-[^/\\]+[/\\]theme[/\\]icons[/\\][^/\\]+\.svg$/,
        use: ['raw-loader'],
      },
      {
        test: /ckeditor5-[^/\\]+[/\\]theme[/\\].+\.css$/,
        use: [
          {
            loader: 'style-loader',
            options: {
              injectType: 'singletonStyleTag',
              attributes: {
                'data-cke': true,
              },
            },
          },
          'css-loader',
          {
            loader: 'postcss-loader',
            options: {
              postcssOptions: styles.getPostCssConfig({
                themeImporter: {
                  themePath: require.resolve('@ckeditor/ckeditor5-theme-lark'),
                },
                minify: false,
              }),
            },
          },
        ],
      }
    ]
  }
};