WebUI Dark Mode: clean up some cr-slider CSS var nits

R=aee@chromium.org
BUG=883049

Change-Id: Ibab66118c2f6e12def7941b43a38651d8dbba914
Reviewed-on: https://chromium-review.googlesource.com/c/1436189
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@{#625981}
1 file changed