As I have not personally experienced this - I did find a closed thread (but with some recent additions, nonetheless) - that may help you with your issue.

https://github.com/webpack-contrib/style-loader/issues/123

The first suggested, and apparently working at the time, solution that sticks out to me is this one:

{ test: /\.css$/, loaders: ['to-string-loader', 'css-loader'] }

There are other suggestions and solutions listed further down; hopefully one of those will help you resolve your problem.

One thing to note - it seems it may/may not make a difference if you're using angular-cli. If you are, then make sure you're following their guidelines with using webpack, as they have a couple small differences, as opposed to using a non-angular-cli build.