WebUI Dark Mode: update cr-slider colors

Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g323037087e_1_0
Screenshots: https://imgur.com/a/NEz2kut

BUG=883049
R=aee@chromium.org

Change-Id: I240adbade23d0be584cb733c3c83acf113968b49
Reviewed-on: https://chromium-review.googlesource.com/c/1435680
Commit-Queue: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625936}
1 file changed