The background is what the remaining elements "sit" on and should always be defined. However
the background can be as small - or as large - as you like and it can have any opacity value you
care to specify.
You can set an option to have the dock hide itself when the mouse cursor leaves it, so you may
wish to set its size so that it accommodates all the Dock Items that are displayed.
The Background has a drop-down menu which you can access by clicking the right mouse
button.
