In a word-processing document, changing a page background also changes the background for all of the pages in its section. To change just one page, make that page its own section and then change its background. In a page layout document, you can change the background of any single page.
Do one of the following:
In a word-processing document: Click a page in the section you want to change, open the Document 
sidebar, then click the Section tab.
In a page layout document: Click an empty spot on the page (so nothing is selected), then open the Format 
sidebar.
Click the disclosure arrow next to Background, then click the pop-up menu and choose one of the following fill types:
A color or gradient designed to go with the template: Choose Color Fill, click the color well to the right of Background, then choose a color or gradient.
A two-color gradient fill: Choose Gradient Fill, then click the color wells below Background and choose colors. The left side of each well shows colors that match the template; the color wheels open the Colors window, where you can choose any color.
A custom gradient: Choose Advanced Gradient Fill, then click the square color wells below the slider to choose colors. Slide the squares and use the other controls to change the blend, angle, and direction of the gradient.
An image: Choose Image Fill, click Choose, then double-click an image file with a .jpg, .png, or .gif filename extension.
