WebUI Dark Mode, Settings: update language page error/explanation colors

Screenshot: https://imgur.com/a/Zo7p0MI

R=dpapad@chromium.org
BUG=926658

Change-Id: I0631c213ad3ef7e5808af1ab9922f00791c60a4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505116
Auto-Submit: Dan Beam (slow; perfing) <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638249}

chrome/browser/resources/settings/languages_page/languages_page.html

@@ -25,6 +25,7 @@
 <link rel="import" href="../settings_page/settings_animated_pages.html">
 <link rel="import" href="../settings_page/settings_subpage.html">
 <link rel="import" href="../settings_shared_css.html">
+<link rel="import" href="../settings_vars_css.html">
 
 <if expr="not is_macosx">
 <link rel="import" href="edit_dictionary_page.html">
@@ -47,6 +48,10 @@
         margin-top: 4px;
       }
 
+      :host-context([dark]) .explain-selected {
+        color: var(--google-green-refresh-300);
+      }
+
       cr-action-menu.complex .dropdown-item {
         min-height: 36px;
       }
@@ -94,13 +99,18 @@
       }
 
       .name-with-error-list div {
+        /* TODO(dbeam): does this need to differ from --settings-error-color? */
         color: var(--google-red-500);
         margin-top: 8px;
       }
 
+      :host-context([dark]) .name-with-error-list div {
+        color: var(--settings-error-color);
+      }
+
       iron-icon[icon='cr:error'] {
         @apply --cr-icon-height-width;
-        --iron-icon-fill-color: var(--google-red-700);
+        --iron-icon-fill-color: var(--settings-error-color);
         margin-inline-end: 8px;
       }