Nimble is an SDK which allows developers quickly and easy create full featured iPhone applications using just HTML and Javascript. This app, NK Blueprint is a demo that showcases the available functions of NimbleKit.
The background image (visible when page is scrolled up or down) is nk_background.png. When an image with this title is added to your Xcode project, Nimblekit will use it as a global background for all webview pages.
If not using nk_background.png you can set a pages background with NKSetWebViewBackgroundColor: eg.
"NKSetWebViewBackgroundColor(1.0, 1.0, 0.0, 1.0);"
will set the pages background to a yellow colour.