You can select from several different pre-loaded backgrounds and themes or use your own background if you'd prefer.
From the menu select Settings --> Appearance --> Theme. You can select a new preloaded background or theme from the dropdown menu at the upper left.
To create your own custom them, tap the “+” icon in the upper left hand corner of the screen. This will duplicate your current theme and allow you to make changes.
To use your own image as a background, tap "Background" in the New Theme setups screen and tap the "+" icon in the upper right to take a photo or pick from existing photos. You can also choose to use a solid color.
There are also settings for modifying the brightness, Icon Set (white or black) and Accent color. Any of these can be applied to a theme you've created or edited. Once you’ve made your changes, you can tap “Rename” in the upper right to name your new creation.
To delete a theme you've created, from the menu select Settings --> Appearance --> Theme. You can also edit your theme by selecting the pencil icon at the top of the screen.