The new style is giving me a lot of grief too. I did figured out enough of the styling to get the black background and bars to look like they used to (no gray wave in the background though).
1: Add this to your XAML that contains the RadChart:
<Style x:Key="StripLineStyle" TargetType="Rectangle">
<Setter Property="Fill" Value="#10FFFFFF" />
2: When you initialize the chart:
this.Resources["StripLineStyle"] as Style;