In the Page Tab click on Background.
Click on the three dots in the right upper corner to open more options.
Choose which pages need a new background.
Click on the circle to choose a color.
Alternatively, you can choose an image file to use as the background for your documents. Click on Image. Click on Browse to select an image file.
Once you have added the image it will appear in the preview section.
Click on Apply at the bottom to change the background.