app_context.js

import React from 'react';

const ThemeContext = React.createContext('light');

class App extends React.Component {
  render() {
    return (
      <ThemeContext.Provider value="dark">
        <Toolbar />
      </ThemeContext.Provider>
    );
  }
}

function Toolbar(props) {
  return (
    <div>
      <ThemedButton />
    </div>
  );
}

function withTheme(Component) {
  return function ThemedComponent(props) {
    return (
      <ThemeContext.Consumer>
        {theme => <Component {...props} theme={theme} />}
      </ThemeContext.Consumer>
    );
  };
}

function Button({theme, ...rest}) {
  return <button className={theme} {...rest} />;
}

const ThemedButton = withTheme(Button);