Try to use thread-loader with mini-css-extract-plugin. When run npm build script, got error:

> cross-env NODE_ENV=production webpack

assets by status 958 bytes [cached] 1 asset
runtime modules 663 bytes 3 modules
cacheable modules 113 bytes
  ./src/index.js 74 bytes [built] [code generated]
  ./src/styles.css 39 bytes [built] [code generated] [1 error]

ERROR in ./src/styles.css
Module build failed (from ./node_modules/thread-loader/dist/cjs.js):
Thread Loader (Worker 0)
You forgot to add 'mini-css-extract-plugin' plugin (i.e. `{ plugins: [new MiniCssExtractPlugin()] }`), please read https://github.com/webpack-contrib/mini-css-extract-plugin#getting-started

    at Object.pitch (D:workspacemrdulinwebpack-sampleswebpack-v5examplesthead-loader-and-mini-css-extract-plugin-issuenode_modulesmini-css-extract-plugindistloader.js:89:14)
 @ ./src/index.js 1:0-34 2:28-34

webpack 5.88.2 compiled with 1 error in 373 ms

Code

// webpack.config.js

const path = require('path');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const os = require('os');

const threadPoolOptions = {
    workers: os.cpus().length - 1,
};
const isProd = process.env.NODE_ENV === 'production';

module.exports = {
    mode: 'production',
    entry: './src/index.js',
    output: {
        path: path.resolve(__dirname, 'dist'),
        clean: true,
    },
    module: {
        rules: [
            {
                test: /.css$/,
                use: [
                    {
                        loader: 'thread-loader',
                        options: threadPoolOptions,
                    },
                    isProd ? MiniCssExtractPlugin.loader : 'style-loader',
                    {
                        loader: 'css-loader',
                        options: {
                            modules: { localIdentName: '[path][local]' },
                        },
                    },
                ],
            },
        ],
    },
    plugins: [new MiniCssExtractPlugin()],
};

index.js:

import styles from './styles.css';
console.log('???? ~ styles:', styles);

styles.css:

.f12 {
    font-size: 12px;
}

package.json:

{
  "version": "1.0.0",
  "scripts": {
    "build": "cross-env NODE_ENV=production webpack"
  },
  "devDependencies": {
    "cross-env": "^7.0.3",
    "css-loader": "^7.1.2",
    "mini-css-extract-plugin": "^2.9.0",
    "style-loader": "^4.0.0",
    "thread-loader": "^4.0.2",
    "webpack": "^5.80.0",
    "webpack-cli": "^5.0.2"
  }
}

• Operating System: win11
• Node Version: v16.20.1
• NPM Version: 8.19.4

Made a minimal, reproducible example here: GitHub Repo