[NTP] Better dark theme detection when dark mode enabled

https://crrev.com/c/1427543 fixed the case where a custom background is
set and dark mode is disabled. However, this does not cover the case
where both a custom background and theme are not set.

Improve dark theme detection by only checking for dark mode if no
custom background or theme is set. Otherwise, default to the regular
dark theme detection.

Bug: 923797
Change-Id: If4f8863cc2a29ed3f8908bbc9d89a7b5ad92a336
Reviewed-on: https://chromium-review.googlesource.com/c/1428440
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625373}
1 file changed