[Cashtab] Update PWA splash screen colors

Summary:
Update bg color of the splash screen displayed when users load the PWA

Android and iOS handle this differently. Apparently there isn't really a way to "push" this kind of update, only for sure works for new users.

Test Plan:
Make sure these are valid hex colors. We can't really test this change before landing.

npm test to ensure we didn't break anything

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17289