Style NTP Wallpaper Search button

*Specs linked in bug's description.

w/o background:
wallpaper search open: screenshot/7XjnW56WE4bRU82
wallpaper search closed: screenshot/4Ppff6zBCuwZDcg

w/ background:
wallpaper search open: screenshot/7fWpVKvwddEEoNy
wallpaper search closed: screenshot/6ruJjnUW4djXKEJ

Change-Id: I294edaab9df386858c616133de2cdd42d12b5abd
Bug: 324631907
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5304249


Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Cr-Commit-Position: refs/heads/main@{#1262042}