open ccsm:
enable Opacity plugin and add
((type=Menu | PopupMenu | DropdownMenu | Tooltip | Notification | Combo | Dnd | name=sun-awt-X11-XWindowPeer)) & !(name=sun-awt-X11-XFramePeer | name=sun-awt-X11-XDialogPeer)
set to 95
http://www.ubuntu1501.com/2007/12/transparencies-and-opacity-in-compiz.html


[...] enable menu transparency [...]