_app.example.js

import NextApp from 'next/app';
import React from 'react';
import { ThemeProvider } from 'styled-components';

const theme = {
  primary: 'green'
};

export default class App extends NextApp {
  // remove it here
  componentDidMount() {
    const jssStyles = document.querySelector('#jss-server-side');
    if (jssStyles && jssStyles.parentNode) {
      jssStyles.parentNode.removeChild(jssStyles);
    }
  }

  render() {
    const { Component, pageProps } = this.props;

    return (
      <ThemeProvider theme={ theme }>
        <Component { ...pageProps } />
      </ThemeProvider>
    );
  }
}