Dark Mode: give user-facing WebUI pages data on whether dark mode is on

R=dpapad@chromium.org
BUG=883049

Change-Id: Ia0657f1333ee3c3e556b36515f97c66a666309d9
Reviewed-on: https://chromium-review.googlesource.com/c/1354685
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614531}
23 files changed