Check the code below...
<ResourceDictionary ...>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/assembly;component/myDic.xaml" />
</ResourceDictionary.MergedDictionaries>
<Style TargetType="{x:Type localui:MyControl}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type localui:MyControl}">
<Button Style="{StaticResource narrowButton}">...</Button>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>