Files app: Fix selection menu bg color when menu is shown

The #selection-menu-button uses <files-toggle-ripple> which sets the
background based on a translucent white background, but because the
button is already white it isn't visible. Setting the background-color
to black, changes the ripple to use a translucent black for which
becomes visible.

Bug: 917326
Change-Id: I3054ee1257ac541f9fd868a2c999abd6ca7a6e96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669130
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
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@{#670834}
diff --git a/ui/file_manager/file_manager/main.html b/ui/file_manager/file_manager/main.html
index 4aff37dd..e99192250 100644
--- a/ui/file_manager/file_manager/main.html
+++ b/ui/file_manager/file_manager/main.html
@@ -78,6 +78,7 @@
             background-color: black;
           };
           --files-toggle-ripple-activated: {
+            background-color: black;
             opacity: 0.08;
           };
         }