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}

chrome/browser/resources/settings/settings_menu/settings_menu.html

@@ -40,7 +40,7 @@
       }
 
       iron-icon {
-        --iron-icon-fill-color: var(--settings-nav-item-color);
+        --iron-icon-fill-color: var(--settings-nav-icon-color);
         margin-inline-end: 24px;
         pointer-events: none;
         vertical-align: top;

chrome/browser/resources/settings/settings_vars_css.html

@@ -28,6 +28,7 @@
       padding-top: 0;
     }
 
+    /* No --settings-nav-icon-color needed in light mode. */
     --settings-nav-item-color: var(--google-grey-refresh-700);
 
     --settings-page-vertical-margin: 21px;
@@ -59,6 +60,7 @@
   }
 
   html[dark] {
+    --settings-nav-icon-color: var(--google-grey-refresh-500);
     --settings-nav-item-color: var(--cr-primary-text-color);
   }
 </style>