How to base a style on another style in a resource dictionary?

I have a theme that is applied to all buttons in a resource dictionary. Now I want to add a trigger to the button while inheriting the style changes from the dictionary. I tried the following code, but it says that the control cannot be found. How can I fix it ?

<UserControl.Resources>
  <ResourceDictionary>
      <ResourceDictionary.MergedDictionaries>
          <ResourceDictionary Source="Theme.xaml"/>
      </ResourceDictionary.MergedDictionaries>

      <conv:ErrorContentConverter x:Key="ErrorContentConverter" />

      <Style x:Key="ValidTrigger" 
             TargetType="Control" BasedOn="{StaticResource {x:Type Control}}">
         <Style.Triggers>
            <DataTrigger Binding="{Binding Path=IsValid}" Value="False">
              <Setter Property="IsEnabled" Value="false" />
            </DataTrigger>
         </Style.Triggers>
      </Style>   
  </ResourceDictionary>
</UserControl.Resources>

The base template:

    <Style TargetType="{x:Type Button}" BasedOn="{x:Null}">
    <Setter Property="FocusVisualStyle" 
            Value="{DynamicResource NuclearButtonFocusVisual}" />
    <Setter Property="Foreground" Value="#FF042271" />
    <Setter Property="FontFamily" Value="Trebuchet MS" />
    <Setter Property="FontSize" Value="12" />
    <Setter Property="Padding" Value="3" />

    <Setter Property="Template" Value="{DynamicResource ButtonTemplate}" />
</Style>