1. 在node_modules下找到react-scripts資料夾

cd node_modules/react-scripts

2. 安裝less、less-loader依賴包

a. yarn安裝

yarn add less less-loader

b. npm安裝

npm install --save less less-loader

3. 在react-scripts/config資料夾找到webpack.config.dev.js和webpack.config.prod.js兩個檔案

{
            test: /\.(css|less)$/,
            loader: ExtractTextPlugin.extract(
              Object.assign(
                {
                  fallback: require.resolve('style-loader'),
                  use: [
                    {
                      loader: require.resolve('css-loader'),
                      options: {
                        importLoaders: 1,
                        minimize: true,
                        sourceMap: shouldUseSourceMap,
                      },
                    },
                    {
                      loader: require.resolve('postcss-loader'),
                      options: {
                        // Necessary for external CSS imports to work
                        // https://github.com/facebookincubator/create-react-app/issues/2677
                        ident: 'postcss',
                        plugins: () => [
                          require('postcss-flexbugs-fixes'),
                          autoprefixer({
                            browsers: [
                              '>1%',
                              'last 4 versions',
                              'Firefox ESR',
                              'not ie < 9', // React doesn't support IE8 anyway
                            ],
                            flexbox: 'no-2009',
                          }),
                        ],
                      },
                    },
                    {
                      loader: require.resolve('less-loader'),
                    },
                  ],
                },
                extractTextPluginOptions
              )
            )