I found no documentation on this yet
Hmmmm...
http://developer.android.com/about/versions/kitkat.html#44-immersive
To get the most impact out of your content, you can now use new window styles and themes to request translucent system UI, including both the status bar and navigation bar. To ensure the legibility of navigation bar buttons or status bar information, subtle gradients is shown behind the system bars. A typical use-case would be an app that needs to show through to a wallpaper.
http://developer.android.com/about/versions/android-4.4.html#UI
You can now make the system bars partially translucent with new themes, Theme.Holo.NoActionBar.TranslucentDecor and Theme.Holo.Light.NoActionBar.TranslucentDecor. By enabling translucent system bars, your layout will fill the area behind the system bars, so you must also enable fitsSystemWindows for the portion of your layout that should not be covered by the system bars.
If you're creating a custom theme, set one of these themes as the parent theme or include the windowTranslucentNavigation and windowTranslucentStatus style properties in your theme.
