You can set it just as any other attached property:
RadChart1.SetValue(StyleManager.ThemeProperty, new VistaTheme());
You can also use the StyleManager.SetTheme() method:
StyleManager.SetTheme(RadChart1, new VistaTheme());
Note, that the Simple theme is not available
for RadChart yet. We plan to deliver it with Q2 release, expected in July.
the Telerik team