WPF ControlTemplate Style GradientStop in Trigger - wpf

Question

Here is my XAML for a TabItem. I want to be able to set the Color of a single gradient stop in a trigger. I know that I can re-define the gradient completely in the trigger's setter, but I want to access a specific property on the background so I can animate it in the future.
I have tried every variation of everything in the trigger's setter and googled for a long time - but I still can't get it to compile. I have also tried class.property syntax, but still nothing. The current error this code raises is:
"Type 'Background.GradientStops[0]' was not found."
I am pretty sure I know what is going on here - and perhaps what I want is impossible. But there has to be a way to animate a control's gradient in a control template...
Can anyone help me?
thanks
<Style TargetType="{x:Type TabItem}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<TextBlock Padding="6 2 6 2" Name="TheHeader">
<TextBlock.Background>
<LinearGradientBrush StartPoint="0, 0" EndPoint="0, 1">
<GradientStop Offset="0" Color="#f4fafd" />
<GradientStop Offset="1" Color="#ceedfa" />
</LinearGradientBrush>
</TextBlock.Background>
<ContentPresenter ContentSource="Header" Margin="0" />
</TextBlock>
<ControlTemplate.Triggers >
<Trigger Property="IsSelected" Value="True">
<Setter TargetName="TheHeader" Property="Background.GradientStops[0].Color" Value="White" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Answer 1

You can animate it, like in the example here.
You also could use a slight hack to set it, though I always prefer creating multiple brushes as resources and swapping them or re-creating a brush in the as you mentioned.
<Style TargetType="{x:Type TabItem}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<TextBlock Padding="6 2 6 2"
Name="TheHeader" Tag="#f4fafd">
<TextBlock.Background>
<LinearGradientBrush StartPoint="0, 0"
EndPoint="0, 1">
<GradientStop Offset="0"
Color="{Binding ElementName=TheHeader, Path=Tag}"/>
<GradientStop Offset="1"
Color="#ceedfa" />
</LinearGradientBrush>
</TextBlock.Background>
<ContentPresenter ContentSource="Header"
Margin="0" />
</TextBlock>
<ControlTemplate.Triggers>
<Trigger Property="IsSelected"
Value="True">
<Setter TargetName="TheHeader"
Property="Tag"
Value="Red" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>