Revert "use oneOf per theme"

This reverts commit e53efb3.

Author: albertstill

packages/react-scripts/config/webpack.config.library.js

@@ -144,9 +144,9 @@ module.exports = {
  ],
  include: paths.appSrc,
  },
- ...cssThemeExtracts.map((themeExtractTextPlugin, index) => {
- return {
- oneOf: [
+ ...cssThemeExtracts.reduce(
+ (accumulator, themeExtractTextPlugin, index) => {
+ return accumulator.concat(
  // The notation here is somewhat confusing.
  // "postcss" loader applies autoprefixer to our CSS.
  // "css" loader resolves paths in CSS and adds assets as dependencies.
@@ -198,10 +198,11 @@ module.exports = {
  ],
  }),
  // Note: this won't work without `new ExtractTextPlugin()` in `plugins`.
- },
- ],
- };
- }),
+ }
+ );
+ },
+ []
+ ),
  {
  // "oneOf" will traverse all following loaders until one will
  // match the requirements. When no loader matches it will fall