theme: add some transparency to the menu backgrounds

They look too hard with a fully opaque black background. A small
bit of transparency will give a softer appearance without impairing
readability.

https://bugzilla.gnome.org/show_bug.cgi?id=694202

data/theme/gnome-shell.css

@@ -128,7 +128,7 @@ StScrollBar StButton#vhandle:active {
 
 .popup-menu-boxpointer {
     -arrow-border-radius: 8px;
-    -arrow-background-color: black;
+    -arrow-background-color: rgba(0,0,0,0.9);
     -arrow-border-width: 2px;
     -arrow-border-color: #a5a5a5;
     -arrow-base: 24px;