pawol.nu/pages/_app.js

import React from 'react'
import PropTypes from 'prop-types'
import {createMuiTheme, ThemeProvider} from '@material-ui/core/styles'
import CssBaseline from '@material-ui/core/CssBaseline'

const darkTheme = createMuiTheme({
  palette: {
    type: 'dark',
    primary: {
      light: '#81c784',
      main: '#4caf50',
      dark: '#388e3c',
      contrastText: '#fff'
    }
  }
})

export default function MyApp(props) {
  const {Component, pageProps} = props

  React.useEffect(() => {
    const jssStyles = document.querySelector('#jss-server-side')
    if (jssStyles) {
      jssStyles.remove()
    }
  }, [])

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

MyApp.propTypes = {
  Component: PropTypes.elementType.isRequired,
  pageProps: PropTypes.object.isRequired
}