The Implicit Style can not be applied to derived controls as their TargetType is different than the one defined in the Implicit Style. If you set the style implicitly, the style is applied only to the types that match the TargetType exactly and not to elements derived from the TargetType value. You can find more information on the topic in the remarks of the Style property
This is why in order to apply a Telerik predefined style on a UserControl deriving from RadRibbonTab, you need to add a Style targeting the UserControl type in the Resources of the application. It is best to define that style after merging the Telerik ResourseDictionaries so that you can base the UserControl Style on the predefined "RadRibbonTabStyle".
Another approach is to add the style directly to the modules of your RadRibbonTabs, like follows:
I hope this information helps.
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