theme: Update message-tray style in the overview

The currently used style works well in "normal" mode, where the
tray is shown to be located beneath the window layer (pushing the
latter up when revealed). It does not work very well in the overview,
so use the same style as dash and workspace switcher there.

data/theme/gnome-shell.css

@@ -1177,6 +1177,11 @@ StButton.popup-menu-item:insensitive {
     height: 72px;
 }
 
+#message-tray:overview {
+    background: rgba(0, 0, 0, 0.5);
+    border-top: 1px solid rgba(128, 128, 128, 0.4);
+}
+
 .notification {
     font-size: 11pt;
     border-radius: 10px 10px 0px 0px;