To reproduce:
The result of the following line is always null:
var test = ThemeResolutionService.GetTheme(ThemeResolutionService.ApplicationThemeName).FindStyleGroup("Telerik.WinControls.UI.RadLabel");
Workaround:
var test1 = ThemeResolutionService.GetTheme(ThemeResolutionService.ApplicationThemeName).FindStyleGroup(new RadLabel());