The RadRibbonDropDownButton's popup is internally wrapped in our custom popup-wrapper which changes the AllowsTransparency
property to true. That is why when you change the default style your change is overridden afterwords.
As a workaround you could listen for the loaded event of the button and set the button popup AllowsTransparency to false, like follow:
sender, RoutedEventArgs e)
var button = sender
var popup = button.FindChildByType<Popup>();
Doing so you do not need to override the default style.
I hope this information helps.
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