The default background is the background template that is set to documents when they are created or open. To change the default background:
From the main window, choose View ▸ Background Settings… to open the Background Settings dialog.
Select the background template you want as default from the list on the left and set its settings on the right.
Press the Set as Default button.
Each time you want to change the default's background settings—such as background color or image file—you must press the Set as Default button to apply the new settings.