[NTP] Support dark mode for add/edit custom link dialog

Screenshots:
https://screenshot.googleplex.com/FqstoCzbOE2.png
https://screenshot.googleplex.com/X0oy2QqeDt9.png
https://screenshot.googleplex.com/tH0us5zzBJM.png

Bug: 919961, 923988
Change-Id: I00bcf543284fb83a2565e9ff73701a07ac279dca
Reviewed-on: https://chromium-review.googlesource.com/c/1428740
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625367}
7 files changed