Dark Mode: stop light mode styles from bleeding into dark mode buttons

Light mode, secondary paper-button:hover border-color was coming through
because an equivalent variable wasn't being defined for dark mode.

It's very hard to see, but I think the same story was happening for
primary paper-button:active box-shadow.

Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g471d243f08_0_869

R=dpapad@chromium.org
BUG=883049

Change-Id: I3db4f2cde037d33086408e980eea43979cc0fe99
Reviewed-on: https://chromium-review.googlesource.com/c/1420304
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624305}
1 file changed