'Sign in with Apple'-button does not follow the Apple Human Interface Guidelines

[tidbeck]

Describe your environment

• Objective C or Swift: Swift
• iOS version: 13
• Firebase SDK version: 6.19.0
• FirebaseUI version: 8.4.2
• CocoaPods Version: 1.9.1

Describe the problem:

App was rejected by Apple with the following explanation:

Guideline 4.0 - Design
Your app uses Sign in with Apple as a login option but does not use the appropriate Sign in with Apple button design, branding, and/or user interface elements as described in the Sign in With Apple Human Interface Guidelines.

Steps to reproduce:

1. Set UIUserInterfaceStyle to Dark in Info.plist
2. Add the Apple auth provider
3. Open the Auth View Controller

Observed Results:

The "Sign in with Apple" button appears as black on black.

Expected Results:

The "Sign in with Apple" button should be white when presented in dark mode.