[iOS] Change TabGroup creation screen text color to Black/White

Make sure the color is Black in light mode and White in dark mode.

Bug: 328348709
Change-Id: I1abc7830738f4e27de7d4ac7e2809c16862edd71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398118


Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Aliona Dangla <alionadangla@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278871}