message: add theme provider
diff: import React from 'react'
  import App, { Container } from 'next/app'
  +import {ThemeProvider} from 'emotion-theming'
  import {
  Layout,
  Pagination
  } from 'mdx-docs'
  import * as components from '../src'
  +import {theme} from '../src'
  import * as docComponents from './doc-components'
  import SideNav from './doc-components/SideNav'
  +import Header from './doc-components/Header'
  const routes = [
  { name: 'Home', path: '/' },
  @@ -30,11 +33,13 @@ export default class MyApp extends App {
  const { Component, page } = this.props
  return (
  + <ThemeProvider theme={theme}>
  <Container>
  <Layout
  components={Object.assign(components, docComponents)}
  {...this.props}
  routes={routes}
  + header={(<Header />)}
  sidebar={(
  <SideNav />
  )}
  @@ -42,6 +47,7 @@ export default class MyApp extends App {
  <Component {...page} />
  </Layout>
  </Container>
  + </ThemeProvider>
  )
  }
  }
  