[NTP] Update gear icon hover color for non-white backgrounds

Bug: 854010
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I5b0d59bb3ab73a6e179a70021779af8afa31e2e2
Reviewed-on: https://chromium-review.googlesource.com/1109029Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569322}

chrome/browser/resources/local_ntp/custom_backgrounds.css

@@ -15,6 +15,10 @@
   background-color: rgb(241, 243, 244);
 }
 
+.non-white-bg #edit-bg:hover {
+  background-color: rgba(255, 255, 255, .1);
+}
+
 #edit-bg-gear:focus {
   outline: 0;
 }