WebUI Dark Mode: fix title text color

This mainly affects section titles and subpage titles in settings.

R=dpapad@chromium.org
BUG=883049

Change-Id: I614cf44adb70becc5ef00479bd19c5372c4929c4
Reviewed-on: https://chromium-review.googlesource.com/c/1429661
Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625371}

chrome/browser/resources/settings/settings_page/settings_subpage.html

@@ -55,7 +55,6 @@
       }
 
       h1 {
-        color: var(--google-grey-refresh-700);
         flex: 1;  /* Push other items to the end. */
         @apply --cr-title-text;
       }

chrome/browser/resources/settings/settings_shared_css.html

@@ -20,7 +20,7 @@
       h2 {
         align-items: center;
         align-self: flex-start;
-        color: var(--google-grey-refresh-700);
+        color: var(--cr-secondary-text-color);
         display: flex;
         font-size: inherit;
         font-weight: 500;

ui/webui/resources/cr_elements/shared_vars_css.html

@@ -71,11 +71,8 @@
   }
 
   html[dark] {
-    --dark-primary-color: var(--google-grey-200);
-    --dark-secondary-color: var(--google-grey-refresh-500);
-
-    --cr-primary-text-color: var(--dark-primary-color);
-    --cr-secondary-text-color: var(--dark-secondary-color);
+    --cr-primary-text-color: var(--google-grey-200);
+    --cr-secondary-text-color: var(--google-grey-refresh-500);
 
     --cr-card-background-color: var(--google-grey-900);
     --cr-card-elevation: {
@@ -95,6 +92,7 @@
     --cr-menu-shadow: rgba(0, 0, 0, .3) 0 1px 2px 0,
                       rgba(0, 0, 0, .15) 0 3px 6px 2px;
     --cr-separator-color: rgba(255, 255, 255, .1);
+    --cr-title-text-color: var(--cr-primary-text-color);
   }
 
   /* Don't use color values past this point. Instead, create a variable that's