<div id="app"></div>

body {
  background-color: #5225bd;
}

#app {
  text-align: center;
}

p {
  color: #FFF;
}

const ThemeContext = React.createContext('light')

class App extends React.Component {
  constructor(props) {
    super(props)
    
    this.state = {
      theme: 'dark'
    }
  }
  
  changeTheme() {
    const actualTheme = this.state.theme
    let newTheme
    if (actualTheme == 'dark') {
      newTheme = 'light'
    } else {
      newTheme = 'dark'
    }
    
    this.setState({
      theme: newTheme
    })
  }
  
  render() {
    const { theme } = this.state
    return (
      <ThemeContext.Provider value={theme}>
        <Button changeTheme={() => this.changeTheme()}/>
        <Value />
      </ThemeContext.Provider>
    );
  }
}

class Button extends React.Component {
  render() {
    return (
      <button 
        onClick={() => this.props.changeTheme()}
        >
        Cambiar theme
      </button>
    )
  }
}

class Value extends React.Component {
  static contextType = ThemeContext;
  render() {
    return(
      <div className="testContainer">
        <p>Theme actual: <strong>{this.context}</strong></p>
      </div>
    )
  }
}
ReactDOM.render(<App />, document.getElementById("app"));