WebUI Dark Mode: update About page update checkmark/error icons

Screenshots: https://imgur.com/a/CoU73B4

R=hcarmona@chromium.org
BUG=926658

Change-Id: If741189597175b1068d7fcd4a8521b997a40a862
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501792
Commit-Queue: Dan Beam <dbeam@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637610}

chrome/browser/resources/settings/about_page/about_page.html

@@ -63,11 +63,11 @@
       }
 
       iron-icon[icon='settings:check-circle'] {
-        fill: var(--google-blue-500);
+        fill: var(--cr-checked-color);
       }
 
       iron-icon[icon='cr:error'] {
-        fill: var(--google-red-700);
+        fill: var(--settings-error-color);
       }
 
       .settings-box .start {