Default background previews should be transparent.

Default background previews should be transparent to allow color
previews on NTP.

Bug: 1009956
Change-Id: Ie4774f61fdc74bc8bcab8a8abf6b6bce643e513b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1842271Reviewed-by: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703044}

chrome/browser/resources/local_ntp/customize.js

@@ -930,7 +930,7 @@ customize.richerPicker_previewImage = function(tile) {
   if (tile.id === customize.IDS.BACKGROUNDS_DEFAULT_ICON) {
     preview.dataset.hasImage = false;
     preview.style.backgroundImage = '';
-    preview.style.backgroundColor = document.body.style.backgroundColor;
+    preview.style.backgroundColor = 'transparent';
   } else if (tile.id === customize.IDS.BACKGROUNDS_UPLOAD_ICON) {
     // No previews for uploaded images.
     return;