styled.jsx

// import ...
import './App.css'; // Import our new CSS

const STRIPE_API_KEY = 'pk_test_...';

const createCharge = token => {
  // ...
};

const Form = props => {
  // Define our custom styles
  const style = {
    base: {
      color: '#32325d',
      fontFamily: '"Open Sans", "Helvetica Neue", Arial, san-serif',
      fontSmoothing: 'antialiased',
      fontSize: '16px',
      '::placeholder': {
        color: '#aab7c4'
      }
    },
    invalid: {
      color: '#fa755a',
      iconColor: '#fa755a'
    }
  };

  const handleCreateCharge = e => {
    // ...
  };

  // Add a `.StripeForm` class to the <form> element
  return (
    <form className="StripeForm" onSubmit={handleCreateCharge}>
      {/* Add our style to the <CardElement /> */}
      <CardElement style={style} />

      <button type="submit">Create Charge</button>
    </form>
  );
};

const StripeForm = injectStripe(Form);

const App = () => {
  // ...
};

ReactDOM.render(<App />, document.getElementById('app'));