[MD settings] change side nav text and icon color

This CL changes the color of the side nav icon and
color text to a slightly lighter grey.

BUG=596143

Review URL: https://codereview.chromium.org/1820533002

Cr-Commit-Position: refs/heads/master@{#382348}

chrome/browser/resources/settings/settings_shared_css.html

@@ -19,11 +19,10 @@
           color: var(--paper-grey-500);
           font-weight: 400;
         }
-        --settings-side-nav-color: var(--google-grey-600);
 
-        /* The title-bar colors use non-MD colors.  These custom colors are
-         * specified by UX design (bettes@).
-         */
+        /* Some colors use non-MD colors.  These custom colors are specified by
+         * UX design (bettes@). */
+        --settings-side-nav-color: rgb(90, 90, 90);
         --settings-title-bar-background-color: rgb(52, 73, 94);
         --settings-title-bar-color: rgb(255, 255, 255);
       }