This needs installation of compizconfig-settings-manager (sudo apt-get install compizconfig-settings-manager) Run (Alt+F2) > ccsm
Under accesibility, there' s an option called Opacity, Brightness and Saturation.
In here, under the Opacity tab, there's a section called "Window specific settings." Click on the New button. Write dropdownmenu inthe Windows textbox. Set the opacity to 85 or so... Click Close, do the same for popupmenu instead of dropdownmenu.