Razzle: Razzle SCSS with CSS Modules

take a look at
https://github.com/natanelia/razzle-afterjs-redux-rnw-example
In that example, the after is using a previous version of razzle (that uses webpack3), but sass related settings work. You would have to remove ExtractTextPlugin because it is not supporting webpack4 (which the new razzle relies on).

Also if you want to take a loot at my updates to that example (for webpack 4), take a loot at below:

const path = require('path');
const resolve=path.resolve;

const autoprefixer = require('autoprefixer');
const fs=require('fs'); 

//var treeify = require('treeify');

const { BundleAnalyzerPlugin } = require( "webpack-bundle-analyzer" );
const paths= require ('razzle/config/paths');

//does not yet work with webpack4
//const InlineChunkManifestHtmlWebpackPlugin = require('inline-chunk-manifest-html-webpack-plugin');

//keep it here,but could not get to work for sass...
const MiniCssExtractPlugin = require("mini-css-extract-plugin");


/* finds an index of babel rules in array of webpack rules */
/*
   {
   'ruleIdx':'',
   'useIdx':''
   }
 */

function getIdx_RulesByLoader (inRulesArr,
                               IS_DEV,
                               IS_WEB,
                               loaderStr,
                               extentionRegExTest) {

  let jsRulesBabelLoaderIdx = -1;
  let useIdx=-1;
  jsRulesBabelLoaderIdx=inRulesArr.findIndex(
    (rule) =>{
      if (rule.test // rule.test is a reg expr 
          && rule.test.exec
          && rule.test.exec(extentionRegExTest)
          && rule.use)
        {
          useIdx=rule.use.findIndex(
            (useItem)=> {
              if (useItem.options
                  && useItem.loader.includes(loaderStr) ){
                return true;
              }
            }
          );
          /*return true if useIdx is a valid index */
          return (useIdx>-1);
        }
    }
  );

  const res={
    'ruleIdx':jsRulesBabelLoaderIdx,
    'useIdx':useIdx
  };

  return res;
}





module.exports = {
  modify(baseConfig, secondArg, webpack) {

    const {target, dev}=secondArg;
    /* make a copy of config */
    const config = Object.assign({}, baseConfig);


    const IS_NODE = target === 'node';
    const IS_WEB = target === 'web';
    const IS_PROD = !dev;
    const IS_DEV = dev; 


    const MYpostCssLoader = {
      loader: 'postcss-loader',
      options: {
        ident: 'postcss',
        sourceMap: IS_DEV,
        plugins: () => [
          autoprefixer({
            browsers: ['>1%',
                       'last 4 versions',
                       'Firefox ESR',
                       'not ie < 9'],
          }),
        ],
      },
    };



    const MYscssLoader = {
      loader: 'sass-loader',
      options: {
        includePaths: [path.resolve(__dirname, './node_modules')],
        sourceMap: IS_DEV
      }};


    const MYcssLoader = IS_NODE ?
                        {loader:"css-loader"}
                       :{
                         loader: "css-loader",
                         options: {
                           minimize: !IS_DEV,
                           sourceMap: IS_DEV,
                           importLoaders: 1,
                           localIdentName:
IS_DEV?'[path]__[name]___[local]':undefined,
                         }
                       };


    /* 
       MiniCssExtractPlugin
       plugin should be used only on 
       production builds without style-loader 
       in the loaders chain, especially if you 
       want to have HMR in development.
     */

    const newSaasRule= {
      test: /\.(sass|scss)$/,
      exclude:[path.resolve(__dirname, './node_modules')],
      use: IS_NODE
         ? [MYcssLoader, MYscssLoader]
         : IS_DEV
         ? [
           'style-loader',
           MYcssLoader,
           MYpostCssLoader,
           MYscssLoader
         ]
          :[
            //'style-loader',
            MYcssLoader,            
            //cannot get this to work      MiniCssExtractPlugin.loader,
            MYpostCssLoader,
            MYscssLoader
          ]
    };//end of newSaasRule


    config.module.rules.push(newSaasRule);


    /* make babel to transply My symlinked directors
       This is relevant to My seetup (others may not have symlinks..)
       and from some directories containing react native modules
       @start@step:1
     */

    /* match with config-override.js on the csr side */
    config.resolve.alias['web_common'] = path.resolve('./src/wc.src');
    config.resolve.alias['app_src'] = path.resolve('./src/app.src');
    config.resolve.alias['rnjs_common'] = path.resolve('./src/js.app');
    //config.resolve.alias.AppStyles$ = path.resolve('./src/app.src/styles/AppStyles')


    //copied from cra config
    //Probably not needed, 
    config.resolve.alias['babel-runtime']=path.dirname(require.resolve('babel-runtime/package.json'));


    /*
     *  add ./src to module resolver so you can import
     *  modules with absolute path
     *  again, this might not be of use to others...
     */
    config.resolve.modules.push('./src');


    let resOfBabelRule=getIdx_RulesByLoader(config.module.rules,
                                            IS_DEV,
                                            IS_WEB,
                                            'babel-loader', //loader name
                                            './testFileNm.js' //test file name for regex
    );



    /* noticed besides my sources, I also had to add react-native-vector-icons
    this was not the only change for RN-vector-icons though, but other changes
    for RN-vector icons are in babel related rules outside of this config file */
    const pathsToMySourcesArr=
      [
        fs.realpathSync(paths.appSrc+'/js.app'),
        fs.realpathSync(paths.appSrc+'/app.src'),
        fs.realpathSync(paths.appSrc+'/wc.src'),
        fs.realpathSync(paths.appNodeModules+'/react-native-vector-icons'),
        fs.realpathSync(paths.appNodeModules+'/react-native-vector-icons/node_modules'),
      ];



    config.module.rules[resOfBabelRule.ruleIdx]
          .include
          .push(...pathsToMySourcesArr);//use spread to push invidiual elements

    //this is a must 
    config.resolve.symlinks=false;

    /* react native vector icons @start@step:1 */

    const newRNVectorIconsRule={
      test: /\.ttf$/,
      loader: require.resolve("file-loader"), // or directly file-loader
      include: fs.realpathSync(paths.appNodeModules+'/react-native-vector-icons')
    };

    config.module.rules.push(newRNVectorIconsRule);

    /* react native vector icons @end@step:1 */


    if (IS_WEB) {
      if ( !IS_DEV ) {
        config.plugins.push( new BundleAnalyzerPlugin( {
          analyzerMode: "static",
          reportFilename: "webpack-report.html",
          openAnalyzer: false,
        } ) );
      }
    }



    /*
     * I share some of my react components with my
     * react native part of the system
     * So  UsefulComponent.web.js   is for web
     *  UsefulCompoenent.android.js is for android
     * Usefulcomponent.ios.js is for IOS
     *
     * By the way, react's CRA allows for web.js convention
     * so this should probably be a feature of razzle too
     * rather than a customization
     */
    config.resolve.extensions = config.resolve.extensions.concat([
      '.web.js'
    ]);

    return config;

  },
};

I have managed to get it to work, thanks for the help. I'll make a PR with example soon.

Could you provide a example @MaxGoh

Bit late to this, but here's what worked for me:

const path = require("path");

module.exports = {
    modify(config, { target, dev }, webpack) {
        for (const rule of config.module.rules) {
            if (rule.test && rule.test.toString() === "/\\.module\\.css$/") {
                const scss = { ...rule };

                scss.test = /\.scss$/;
                scss.include = path.join(__dirname, "src");
                scss.use.push({ loader: "sass-loader" });

                config.module.rules.push(scss);

                break;
            }
        }

        return config;
    }
};