The Background Tool can be opened by clicking on the following icon.

The Background Tool has 3 main options. 

Apply a flat colour as the canvas background.

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Apply a grid as the canvas background. This can be helpful to align or to measure them moving or resizing elements. The grid option comes with two sizes. 25mm/1" or 50mm/2".

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Apply an image as the canvas background. The tool will show you any images that are in the special category called Backgrounds in the Images library.