WebUI Dark Mode: fix disabled {cr,settings}-slider contrast for a11y

Screenshot: https://imgur.com/a/ftEBVpS

BUG=953005

Change-Id: Ife36ae9492bdc8d1a22c10dce1f13ea34753998a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578752
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654183}