Absolutely... we made our renderers so you can design your own from scratch or simply change the properties of ours. NavigationBarHeaderBackgroundFill is a BackgroundFill type so you can create any sort of BackgroundFill to set to that property. Some examples are SolidColorBackgroundFill, TwoColorBackgroundFill, ImageBackgroundFill, etc. All of them are in the ActiproSoftware.Drawing namespace.
When you run the NavigationBar sample, you can click the Renderer tag, select a BackgroundFill property, and click the ... button to see a designer window that shows the BackgroundFill in detail with a preview and allows you to select other types of BackgroundFills.