Solved + more details:
1. I am using implicit themes and the noxml assemblies
2. The Application merged dictionaries are cleared and loaded from one of the resource assemblies in UI for WPF Q1 2015
3. XAML is NOT used in the application for window definitions, everything is done with code - except where DataTemplates are useful - as in this case for styling as a resource
Solution:
The resource used to replace the MenuItemStyle in MUST reference a base - BasedOn="{StaticResource RadMenuItemStyle}" - to enable it to render as it completely replaces the current resource in the merged dictionary in Application.Current.Resources.
<?xmlversion="1.0"encoding="utf-8"?> <ResourceDictionaryxmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" xmlns:local="clr-namespace:Anvil.Desktop.Windows8"> <Stylex:Key="MenuItemStyle"TargetType="telerik:RadMenuItem"BasedOn="{StaticResource RadMenuItemStyle}"> <SetterProperty="Header"Value="{Binding Text}"/> <SetterProperty="ItemsSource"Value="{Binding Items}"/> <SetterProperty="Icon"Value="{Binding IconUrl}"/> <SetterProperty="IconTemplate"> <Setter.Value> <DataTemplate> <ImageSource="{Binding}"Stretch="None"/> </DataTemplate> </Setter.Value> </Setter> </Style></ResourceDictionary>