You'll need to use CSS to override design defaults. General details can be found here.
The precise CSS implementation may differ depending on your exact requirements.
For example, to change the background images of the screen and navigation bar, from the default striped pattern, you can override the background-image CSS property of body and ul.navbar (both defined in 004-styles.iphone.css), by adding the following to web/stylesheet.css:
Similarly, overriding the default button text color, would be achieved via:
Hope this gives you a starting point with regards to overriding the default design.
For best results, use the Firefox browser..