Dark Mode: update cr-toast colors

Also moves some duplicate color styles into cr-toast itself (and uses
::slotted(*) to apply).

Screenshots: https://imgur.com/a/JXx291d
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g48a476f4a5_2_63

R=dpapad@chromium.org
BUG=883049

Change-Id: I983b7cf9d6b63b5b45a2f8897f7a9cdf6d1de244
Reviewed-on: https://chromium-review.googlesource.com/c/1425967
Auto-Submit: Dan Beam <dbeam@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624866}
8 files changed