## Enable HMR for CSS Current webpack settings uses `mini-css-extract-plugin` for both development and production. This plugin extract css into many small files based on the modules that imports them.
However, per the package's README, it is recommended for production only especially if we want Hot Module Replacement (HMR) for CSS. To use HMR, we need to use `style-loader` when in development mode. Otherwise, the page will not update after saving CSS files. https://github.com/webpack-contrib/mini-css-extract-plugin#advanced-configuration-example To ensure that `style-loader` works correctly, I also need to modify the html templates to import `theme` js entry points. Because `style-loader` injects css code via js that writes `<style>` tags instead of exporting css files. ## Optimize CSS output Current CSS output from webpack are not minified. This PR adds `optimize-css-assets-webpack-plugin` to minify CSS outputs. The minified results reduce CSS size by ~100kb  ## Testing I have tested building in production and running dev server. Both are working fine. @graceguo-supercat @xtinec @hughhhh [ Full content available at: https://github.com/apache/incubator-superset/pull/5813 ] This message was relayed via gitbox.apache.org for [email protected]
