WebUI Dark Mode: fix settings side bar icon color

In dark mode, the *icon* color differs slightly from the *text* color.

Screenshots: https://imgur.com/a/asWpYhY
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g48a476f4a5_2_42

R=dpapad@chromium.org
BUG=883049

Change-Id: I67521d7505f1b27f212b4813c853074f88c28971
Reviewed-on: https://chromium-review.googlesource.com/c/1429196
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625374}
2 files changed