There is nothing that can be done about this right now. There are many postings on here dealing with "white background", yet they all have to do with the same thing. Nimblekit does not display like a navite obj-c application does. Nimblekit uses the UIWebview, which is basically the safari browser engine. It is like making a Web App with the ability to access native controls. The white you are seeing is the default appearance of the browser when nothing is loaded into it, and that is the same on the iPhone or on any other mobile device or computer (Mac, PC, Linux, Palm, Blackberry, Windows Mobile, etc). There is nothing loaded into the browser yet, so your background image or hex color can not be seen until the browser object (which appears white) loads it into memory. White is actually a representation of "nothing" that the browser gives you rather than an actual color. White means, blank, transparent, empty, nothing there in the Browser world. It is not like Nimblekit is setting the screen to white... it just is.
Sunny is looking into a way to work around this, however, I wouldn't expect this to occur until v1.6 or later. It would most likely involve caching browser data in another Webview instance that is hidden and maybe copying the hidden instance to the active instance, or possibly something completely different. Either way, I wouldn't expect this to be fixed right away, so if you are waiting on this to submit an app, you have a tough decision to make... wait for Nimblekit to address this or submit to the App store and make the change as an update to your App when this feature is available.