[NTP] Set up dark mode config, and apply dark mode background when enabled

Set up the dark mode config to be used in all NTP components. Also apply
a dark background for the default NTP when dark mode is enabled.

Default NTP: https://screenshot.googleplex.com/DtOZNv99hfd.png
Third-party NTP: https://screenshot.googleplex.com/kOcDzLmwTmj.png

Bug: 919961
Change-Id: I253cb6655493a5497dcba1fcf16bec4a4e4d19bc
Reviewed-on: https://chromium-review.googlesource.com/c/1407951
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622540}
2 files changed