next.js/examples/with-app-layout/pages/_app.js

import React from 'react'
import App, { Container } from 'next/app'

class Layout extends React.Component {
  render () {
    const { children } = this.props
    return <div className='layout'>{children}</div>
  }
}

export default class MyApp extends App {
  render () {
    const { Component, pageProps } = this.props
    return (
      <Container>
        <Layout>
          <Component {...pageProps} />
        </Layout>
      </Container>
    )
  }
}