WebUI Dark Mode: fix settings subpage backgrounds

TBR=dpapad@chromium.org
BUG=883049

Change-Id: Icdef0838145fbc7c207d1c3d57b9cb4d13d1cbb4
Reviewed-on: https://chromium-review.googlesource.com/c/1429759
Reviewed-by: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Commit-Queue: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625118}
1 file changed