Dark Mode: fix warning color

Screenshots: https://imgur.com/a/NcdRQk1 (less bright is correct)

R=rbpotter@chromium.org
BUG=883049

Change-Id: I764f25d64c7f3b9766a2d9abfeb315b9b928fded
Reviewed-on: https://chromium-review.googlesource.com/c/1403987
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621763}
2 files changed