Wild Book uses WordPress Customizer API, this makes the theme very easy to configure. The Customizer is a framework for live-previewing any change to WordPress. It provides a simple and consistent interface for users to customize various aspects of their theme and their site, from colors and layouts to widgets, menus, and more.
Just go to Appearance > Customize in your WordPress Admin Panel, and there you can find all the customization options.
Sidebar Background Image