React: Gérer l'état localement d'abord: CSS pb

Hello,
Gérer l’état localement d’abord

I had succed with this exercise.
I just have a problem with css of the submit button. it’s very small.
I tried to use ‘btn btn-default’ of Bootstrap but doesn’t work.

Normally I think button element has own default css.
What can I do to improve this ?

class DisplayMessages extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      input: '',
      messages: []
    }
    this.handleChange = this.handleChange.bind(this);
  }
  // Add handleChange() and submitMessage() methods here
  handleChange = (event) => {
    this.setState({
      input: event.target.value
    })
  }
  submitMessage = () => {
    //const tabMess = this.state.input
    this.setState({
      messages: [...this.state.messages, this.state.input],
      input: ''
    })
  }
  render() {
    console.log(this.state)
    const items = this.state.messages.map(item => <li>{item}</li>)
    return (
      <div>
        <h2>Type in a new Message:</h2>
        { /* Render an input, button, and ul below this line */ }
        <input value={this.state.input} onChange={this.handleChange} />
        <button onClick = {this.submitMessage} className="btn btn-default"/>
        <ul>{items}</ul>

        { /* Change code above this line */ }
      </div>
    );
  }
};

You could give the button some text:

<button
  onClick={this.submitMessage}
  className="btn btn-default"
>
  Submit
</button>

You could also style it.