Press the '+' icon next to the Add Screen node to add a new screen.
The following dialog window will appear.
Select the Solid tab to choose a solid background color for the new screen.
Select the Gradient tab to choose a 2-color gradient background for the new screen.
Select the From File tab to use an image file for the new screen's background. Image files can be created in Photoshop or some other image editor.
Select the From Clipboard tab to use a screen capture as the new screen's background.
An alias can also be given to the screen. This alias can be used elsewhere in the editor to refer to the screen.