Gatsby page example

index.js

import React from 'react'

export default class IndexPage extends React.Component {
  // don't do this 😈
  componentDidMount() {
    const node = document.getElementById('whatever')

    node.style.color = 'blue'
  }
  render() {
    return (
      <div id="whatever">
        <p>I am blue when mounted</p>
      </div>
    )
  }
}