webpack.css.config.js

const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const postcssPrefixer = require('postcss-prefixer');
...
const isDevServer = process.env.DEV_SERVER === 'true';
...
module: {
  rules: [
    ...
    {
      test: /\.s[ac]ss$/i,
      use: [
        isDevServer ? 'style-loader' : MiniCssExtractPlugin.loader,
        {
          loader: 'css-loader',
          options: {
            sourceMap: true
          }
        },
        {
          loader: 'postcss-loader',
          options: {
            sourceMap: true,
            plugins: [
              postcssPrefixer({
                prefix: 'tui-full-calendar-'
              })
            ]
          }
        },
        {
          loader: 'sass-loader',
          options: {
            sourceMap: true
          }
        }
      ]
    },
    {
      test: /\.(gif|png|jpe?g)$/,
      use: 'url-loader'
    }
  ]
},
plugins: [
  ...,
  new MiniCssExtractPlugin({
    filename: `${FILENAME}.css`
  })
]
...