[files] Update file manager's background to semantic color.

Update the background to use semantic color. This doesn't change the
currently supported light color scheme but makes it compatible to some
of the texts (like banner and dialog) that use semantic colors, so they
will be visible when testing dark color scheme.

Also updating the file label color to semantic one for the same
purpose.

Bug: 1096951
Change-Id: I8d891181784b0bdac4c9430355738281d4938296
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2251642
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781282}

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

@@ -78,7 +78,7 @@ a:focus {
 }
 
 body.files-ng .dialog-container {
-  background-color: rgb(255, 255, 255);
+  background-color: var(--cros-default-bg-color);
 }
 
 /* List and grid are inside this container. */
@@ -3192,7 +3192,8 @@ body.files-ng .table-row-cell:first-child > * {
 }
 
 body.files-ng .table-row-cell .filename-label {
-  color: var(--google-grey-900);
+  color: var(--cros-default-text-color);
+
   padding-top: 0;
 }