Background settings are found in the Design editor, clicking on the Settings button next to the slide. There, you'll find the following controls: Color Set, Background Style and Background Image. 

Changing Color Set will apply a different color variation to the slide background and elements, alternating the color combinations available in the palette. 

There are different Background Styles you can apply, with gradient and shadowing options. 

Background Images can be added by just clicking the "Select an Image" button. You can browse the built-in image gallery by just typing in keywords to find related photos, or upload your images. GIF animations and Icons can be used as backgrounds too. 

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