app_redux.js

import React from "react";
import { createStore } from "redux";
import { Provider, connect } from "react-redux";

function theme(state, action) {
  return {
    theme: "dark"
  };
}

const store = createStore(theme);

class App extends React.Component {
  render() {
    return (
      <Provider store={store}>
        <Toolbar />
      </Provider>
    );
  }
}

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

const mapStateToProps = state => {
  return {
    theme: state.theme
  };
};

const ThemedButton = connect(mapStateToProps)(props => (
  <button theme={props.theme} />
));