You can change the background of an entire page by clicking the [Body Settings] from the top left corner of the page. 

   
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Change the Background Color

You can pick a solid color for the background, use the color theme generator, or paste in a hex color code.

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Change the Background Image

You can also upload an image from your computer or paste in a URL that points to that image.

You can set the position of the background image to cover the entire page or just have it contained inside the page.

You can set the image to tile in the background.