The horizontal scrollbar disappears setting RadTileList's background to black and when Windows8Touch theme is used.

Declined: By default the ScrollBar's Thumb in Windows8Touch theme is transparent black and it is expected not to be visible on black background.

For customization of the ScrollBar see attached project:
1. Extract the Style and Control Templates for the ScrollBar and its components from the System.Windows.xaml
2. Modify the HorizontalThumb and VerticalThumb's backgrounds to any color different from black - e.g. MainBrush, which is white in Windows8Thouch theme, change any needed opacities in the VisualStates
3. Apply the style globally for your application in App.xaml or only to the needed controls - in their <Control>.Resources (including all its needed resources)