This is very very crazy.
I have boiled down the code to just this:
Only thing changing being the ItemsSource bind value and my grid is still stuck at default theme :/ While yours uses the theme. I have tried other themes as well but alas, to no avail.
FYI: Data is fine in both cases.
My Code just in case:
I have also upgraded the Telerik controls (inc. references), cleaned, rebuilt the project - Still nothing. Please note I am using the theme successfully in other parts of the project.
Unfortunately I can not send my code as the project is too large but: Other than that XAML code, are there any steps to be taken for this to work?