Change expander background on mouse hover - wpf

Question

I would like to change the expanders background color to a lighter variant on mouse hover. So I figured I use a trigger and a converter to convert the background color to a lighter variant on mouse over. I started simple and only implement the trigger. That works:
<StackPanel>
<StackPanel.Resources>
<Style TargetType="{x:Type Expander}">
<Style.Triggers>
<Trigger Property="Control.IsMouseOver" Value="True">
<Setter Property="Background" Value="Red"/>
</Trigger>
</Style.Triggers>
</Style>
</StackPanel.Resources>
<Expander Header="Header">
<StackPanel>
<TextBox Background="Transparent">Content</TextBox>
</StackPanel>
</Expander>
</StackPanel>
So I can achieve almost what I want. But I want to be able to set the color on the expander like this:
<Expander Header="Header" Background="Yellow">
The moment I add that the color switching stops working and the expander is always yellow. Why is that happening and how can I achieve my goal?
Edit: in my final application I will have an itemscontrol with multiple expanders whose background color is databound to my view model, so I think I cannot set the color in the style.
<ItemsControl ItemsSource="{Binding TestStepDescriptions}">
<ItemsControl.Resources>
<DataTemplate DataType="{x:Type localVMsTestEngines:AutomaticTestStepDescription}">
<Expander Background="{Binding StepStatus, Converter={StaticResource StepStatusToColorConverter}}">
regards,
Jef

Answer 1

You cannot set that color in the Expander directly if you use a Trigger.
Instead you have to do it like this:
<ItemsControl ItemsSource="{Binding TestStepDescriptions}">
<ItemsControl.Resources>
<DataTemplate DataType="{x:Type localVMsTestEngines:AutomaticTestStepDescription}">
<Expander >
<Expander.Style>
<Style TargetType="{x:Type Expander}">
<Setter Property="Background" Value="{Binding StepStatus, Converter={StaticResource StepStatusToColorConverter}}"/>
<Style.Triggers>
<Trigger Property="Control.IsMouseOver" Value="True">
<Setter Property="Background" Value="Red"/>
</Trigger>
</Style.Triggers>
</Style>
</Expander.Style>
</Expander>
Usage
<Expander Header="Header">