[OOBE] Polish backdrop

Bug: b:269104473
Change-Id: I884eef4f0521738c6a774fca645a8e5d1b97cae9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4528236


Commit-Queue: Simon Ziegltrum <ziegltrum@google.com>
Reviewed-by: Danila Kuzmin <dkuzmin@google.com>
Cr-Commit-Position: refs/heads/main@{#1146409}