Files app: Add white background for FilesNG

Set white background in the top level container and forces directory
tree and file list to inherit it.

Bug: 992821,992819
Change-Id: I610ec28fe06a092ada41ad268a18f3495d46b8be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026552
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736244}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -73,6 +73,10 @@ a:focus {
   position: relative;
 }
 
+body.files-ng .dialog-container {
+  background-color: rgb(255, 255, 255);
+}
+
 /* List and grid are inside this container. */
 .dialog-main {
   align-items: stretch;
@@ -82,6 +86,10 @@ a:focus {
   flex-direction: column;
 }
 
+body.files-ng .dialog-main {
+  background-color: inherit;
+}
+
 /* Directory tree at the left. */
 .dialog-navigation-list {
   background-color: rgb(250, 250, 250);
@@ -96,6 +104,10 @@ a:focus {
   width: 240px;
 }
 
+body.files-ng .dialog-navigation-list {
+  background-color: inherit;
+}
+
 .dialog-navigation-list-contents {
   display: flex;
   flex: 1 1 auto;