If there is unused space on the canvas, the background color by default becomes black. You can edit the background color by clicking Setup within the editor in the bottom menu bar and then selecting your color of choice:
Another way of editing the background color or design of a design is to create a graphic image (using a site such as http://Canva.com) that spans the entire canvas space.
Below are the recommended dimensions for each design size:
Square: 1080 x 1080
Vertical: 720 x 1280
Horizontal: 1280 x 720
For example, if you were working with the square design size and wanted a plain white background, you could create a 1080 x 1080 image that includes your logo and has a white background.