Open Files as Administrator.
Enter your password....
Navigate to the folder /usr/share/themes/elementary/gtk-3.0/
Find the file apps.css and open it ( scratch-text-editor ).
Scroll down to the following section :
Just below is the line : background-color: alpha (#000, 0.7);
#000 is the color ( see : http://www.color-hex.com/ ), 0.7 is the transparency.
Change the number 0.7 into a lower number ( e.g. 0.3 ).
To see the change type in a terminal :
In order to make the slingshot-launcher and the drop down menus from the panel transparent, we need to change another file :
sudo nano granite-widgets.css
Type Ctrl+W ( Where Is ) and type popover_bg and hit Enter.
There are three lines containing alpha and we need to change the last numbers ( 0.97 ) in them ( make them lower ). So it will look like this :
To see the changes :