feat: add styles to fix braintree modal appearance

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR will be ready to go after Automattic/newspack-plugin#3692 is merged. It makes Braintree's credit card form look closer to other payment processors.

How to test the changes in this Pull Request:

1. Start on a test site running Braintree, and that the Braintree credit card is enabled as a payment processor.
2. Apply this PR and run npm run build.
3. Run through a test checkout and confirm that the Braintree credit cart form looks similar to Stripe and other payment gateways:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?