trouble with tab control header info

when applying a control template to a tab item I'm struggling to access the
tab item header content vs the tab item content. I finally figured out to
set the controlpresenter source="Header" to get the content to come thru.
I'm now trying to change the font attributes of the header content when the
tab is selected. The fontsize setter in the trigger below changes all font
sizes not just the header. Thanks

<Style x:Key="styleRoundTab" TargetType="{x:Type TabItem}">
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0,0.5" EndPoint="1,0.5">
<LinearGradientBrush.RelativeTransform>
<TransformGroup>
<TranslateTransform X="-0.5" Y="-0.5"/>
<ScaleTransform ScaleX="1" ScaleY="1"/>
<SkewTransform AngleX="0" AngleY="0"/>
<RotateTransform Angle="270"/>
<TranslateTransform X="0.5" Y="0.5"/>
<TranslateTransform X="0" Y="0"/>
</TransformGroup>
</LinearGradientBrush.RelativeTransform>
<LinearGradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="sc#1, 0.10, 0.14, 0.08" Offset="1"/>
<GradientStop Color="sc#1, 0.0002002716, 0, 1" Offset="0"/>
</GradientStopCollection>
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="Foreground" Value="White" />
<Setter Property = "FontWeight" Value="Regular"/>
<Setter Property="Width" Value="150" />
<Setter Property="Height" Value="50" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<Grid Height="{TemplateBinding Height}">
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
<Rectangle Grid.Column="0" Width="140" RadiusX="33" RadiusY="13"
Margin="0,0,0,0" Fill="{TemplateBinding Background}">
</Rectangle>
<Rectangle Grid.Column="1" Grid.ColumnSpan="2" Width="100"
HorizontalAlignment="Center" Margin="0,0,0,0" Fill="{TemplateBinding
Background}">
</Rectangle>
<ContentPresenter Grid.ColumnSpan="2" Grid.Column="0"
Margin="13,2,13,4" ContentSource="Header" HorizontalAlignment="Center"
VerticalAlignment="Center" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsSelected" Value="true">
<Setter Property = "Height" Value="55"/>
<Setter Property = "FontSize" Value="15"/>
</Trigger>
</Style.Triggers>
</Style>