Server side rendering is a large problem of its own to solve, but essentially you need to use universal-webpack and extract-text-webpack-plugin to extract out your CSS and <link /> to it in your server. I remember the major issue that I had was not being able to get Node to recognize my import style from './index.css' statements, but I think universal-webpack solved that for me. I also recommend you check out Next.js.
Marcus Pohorely
CTO/CEO @Pagelanes.com