MD Settings: darken secondary text gray

chrome/browser/resources/settings/settings_shared_css.html

 <!-- Common styles for Material Design settings. -->
 <dom-module id="settings-shared">
   <template>
     <style>
       :root {
         --checkbox-margin-start: 2px;
         --checkbox-size: 16px;
         --checkbox-spacing: 18px;
         --iron-icon-fill-color: var(--paper-grey-500);
         --iron-icon-height: 20px;
         --iron-icon-width: 20px;
         --paper-icon-button: {
           width: 36px;
           height: 36px;
         }
         --settings-background-color: var(--paper-blue-grey-50);
         --settings-secondary: {
-          color: var(--paper-grey-500);
+          color: var(--paper-grey-600);
           font-weight: 400;
         }
 
         /* Some colors use non-MD colors.  These custom colors are specified by
          * UX design (bettes@). */
         --settings-nav-grey: rgb(90, 90, 90);
         --settings-title-bar-background-color: rgb(52, 73, 94);
         --settings-title-bar-color: rgb(255, 255, 255);
       }
 
@@ skipping 213 matching lines @@
 
       .settings-checkbox-spacer {
         -webkit-margin-start: calc(
             var(--checkbox-margin-start) +
             var(--checkbox-size) +
             var(--checkbox-spacing));
       }
     </style>
   </template>
 </dom-module>