The property you need to set is:
public partial class App : Application
Telerik.Windows.Controls.Theme.ApplicationTheme = "Telerik.Windows.Themes.Vista";
This will set application-wide theme for ALL Telerik controls. Note that if you want to enable custom made application-wide theme you should put all XAML files in a folder named "themes" and also the resources for each control should be in separate XAML file, named after the control, e.g.
If a control is unable to find a needed resource in your theme XAML it will automatically use the same resource from its default theme.
I hope this helps.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.