Element-UI's components don't display when running 'npm run build' 

Would you please tell me how to use Element-UI in the content?

I want to use Element-UI in the content_script; src/content/index.js.
This trial goes well when I run 'npm run dev'.
However it doesn't go well when I run 'npm run build'.
Because the webpack output the CSS data to the files as 'src/css/foo.css' when only the 'build'.