Change the slide background in Keynote for iCloud
You can add an image, a solid color, or a gradient (two colors that blend into one another) to the background of a slide.
Select a slide thumbnail in the slide navigator (on the left).
In the Format sidebar on the right, make sure the Background checkbox is selected, then choose one of the following fill types:
A preset color or gradient designed to match the current theme: Click the preset color well to the right of the checkbox, then choose a color or gradient.
A custom color or gradient: Click the pop-up menu below the checkbox and choose Color Fill or Gradient Fill. Click the custom color well below the pop-up menu (it has more options than the preset color well) and choose a color.
An image: Click the pop-up menu below the checkbox, then choose Image Fill. Click Choose, then double-click an image file with a .jpg, .png, or .gif filename extension.
If objects on the page, such as table cells, charts, and text boxes, don’t have their own background fill, they show the slide’s background color. To fill the object with a different color, see Change the background of table cells, Change the chart background, or Fill objects with color or an image.