I am pasting the reply from the support ticket:
I would suggest to use implicit styles in order to apply the Windows8 (the new name of the Metro theme) theme to RadListBox ( and any other RadControls that you're using). This will make the customizations much easier. Detailed information on the implicit styles can be found here.
Have in mind that the style for the ScrollBar is defined in System.Windows.xaml and it's important whether you need to customize it only in RadListBox or in general.
the Telerik team