property Ribbon.PopupVisibility as Long
Specify the popup's visibility in percents: 90% is barely visible, and 10% is nearly opaque.

TypeDescription
Long A long expression that indicates the visibility of the popup menus.
By default, the Visibility is 100. Use the Visibility property to change the menu's visibility.

The following screen shot shows the menu when the Visibility is 100 ( opaque, by default ):

The following screen shot shows the menu when the Visibility is 80 ( semi-transparent ):