Click outside the format to unselect your layers (if you have selected one)
Select the format background color option
Monochrome - one color
Gradient - 2 colors with 3 gradient directions available
💡 Tip : You want a transparent background? Set the opacity to 0 in the alpha "A" value and voilà, your background color is transparent. (your visual will be generated as a .png instead a .jpg)