WebUI Dark Mode: update checkmark color on import bookmarks & settings

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

R=dpapad@chromium.org
BUG=926658

Change-Id: I26bca3932d0e92dad7d6c3cbfab35fe543e022f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1501992
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637596}

chrome/browser/resources/settings/people_page/import_data_dialog.html

@@ -29,7 +29,7 @@
       }
 
       #successIcon {
-        fill: var(--paper-blue-600);
+        fill: var(--cr-checked-color);
         height: 80px;
         margin: auto;
         width: 100%;