The Page Body Element is the background of your entire page. As a default you page background is white, but you can change the background color or add a background image.
To open the Body Settings, double click on the body or right click, to show the settings option.
You can also find the body settings icon at the upper left corner of the page, this can be clearly seen when you add a background color or background image.
If you have other elements in the page and you're having a hard time selecting the body element settings from the upper left corner, just click body from the lower left corner of your screen to select the body element. Then double click or right click any portion in the page where there is no element on top of the body yet to show settings option.