p13n: thumbnails loading text color update

Increase legibility by switching to using on_primary token.

Before (Light): https://screenshot.googleplex.com/AWminceqdG9Tdfk
After (Light): https://screenshot.googleplex.com/5Rzstotc4iNqaVH
Before (Dark): https://screenshot.googleplex.com/8pTv6w8GqwoqyhR
After (Dark): https://screenshot.googleplex.com/56GdTQzwJvHZe9f

BUG=b:324246783
TEST=Manually

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: Icf6268bf29f0fa1a2728d7f977a43cd4ffc12911
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402539


Reviewed-by: Jeffrey Young <cowmoo@google.com>
Commit-Queue: Jeffrey Young <cowmoo@google.com>
Cr-Commit-Position: refs/heads/main@{#1280247}