Share extension buttons are not visible in light mode

(Originally filed here → https://github.com/wordpress-mobile/WordPress-iOS/issues/14157)

When using WordPress iOS with light mode on, the share extension's `Cancel` and `Save` buttons have white text. The buttons still work, they are just not visible. The buttons _are_ visible in dark mode:

When testing this in other apps (e.g. Safari), everything displays correctly.

### To repro

1. In Settings.app, turn on light mode
2. In WPiOS, initiate share to Simplenote. (e.g.: on a post, tap the Share nav bar button and select Simplenote)
3. When the share view appears, notice cancel and Save are hidden (white text...buttons still work).

_Simplenote 4.19
iPhone 11 Pro
iOS v13.5.1_