[Mobile Payments] Preparing reader spinner on iPad has black bars at the top and bottom in dark mode

[joshheald]

Description

When the Preparing reader modal shows before a payment on an iPad, it shows black bars at the top and bottom when dark mode is on.

These bars should not be visible

Repro

1. On an iPad with dark mode enabled
2. Navigate to Menu > Payments > Collect Payment
3. Go through the payment flow and select Card on the payment method screen
4. Wait to connect to a reader: once this is done, you will briefly see the Preparing reader screen
5. Observe that there are two black bars at the top and bottom of the spinner

Expectation: the background should be a consistent colour across the modal view.