App.js

import React from "react";
import styled from "styled-components";

const Container = styled.div`
  display: flex;
  flex-direction: column;
  width: 100vw;
  height: 100vh;
  align-items: center;
  justify-content: center;
  font-family: sans-serif;
`;

const App = () => (
  <Container>
    <h1>My Dark Theme Is Better Than Yours</h1>
    <p>
      <input type="checkbox"></input> Use Dark Theme
    </p>
  </Container>
);

export default App;