When nothing is selected, you can edit the background in the right toolbar.
1. Pick one of the common colors, or choose any color from the color palette. You can also use theme colors, which are the colors used most often in the objects on your canvas.
2. You can also upload an image as the background. The image is cropped to the actual print size, so only the cuttable area shows on the canvas. This does not affect the printed result.