[NTP] Consolidate button style to common CSS and add dark mode support

Consolidate paper button styling to animations.css. Also add dark mode
support.

Screenshots (Dialog background has been set to dark only for
screenshots):
Enabled = https://screenshot.googleplex.com/rOW5AuTnunN.png
Disabled = https://screenshot.googleplex.com/QJrCHor6bHS.png
Active/hover = https://screenshot.googleplex.com/vWLs9D24F39.png

Bug: 919961
Change-Id: Iceb2741de72c4cfea67ce5e6bb20662a309b530d
Reviewed-on: https://chromium-review.googlesource.com/c/1427246
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625068}
6 files changed