How to change the Property="Style" in trigger - wpf

Question

Trigger is not working. IsMouseOver is not changing style of the Button. I am not sure what is wrong with my code. I am trying to change the style property of the button on mouse over property. It keeps throwing error.
Basically I am trying to use different style on mouse over event
<Style x:Key="FancyButtonStyle" TargetType="{x:Type Button}">
<Setter Property="FontSize" Value="{Binding FontSize}"/>
<Setter Property="Foreground" Value="{Binding Foreground}"/>
<Setter Property="FontWeight" Value="{Binding FontWeight}"/>
<Setter Property="Width" Value="{Binding Width}"/>
<Setter Property="Height" Value="{Binding Height}"/>
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
<GradientStop Offset="0.0" Color="White" />
<GradientStop Offset="0.2" Color="DarkGray" />
<GradientStop Offset="0.6" Color="Black" />
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Style" Value="{StaticResource ResourceKey=FancyButtonInvertStyle}"/>
</Trigger>
</Style.Triggers>
</Style>
<Style x:Key="FancyButtonInvertStyle" TargetType="{x:Type Button}">
<Setter Property="FontSize" Value="{Binding FontSize}"/>
<Setter Property="Foreground" Value="{Binding Foreground}"/>
<Setter Property="FontWeight" Value="{Binding FontWeight}"/>
<Setter Property="Width" Value="{Binding Width}"/>
<Setter Property="Height" Value="{Binding Height}"/>
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
<GradientStop Offset="0.0" Color="Black" />
<GradientStop Offset="0.4" Color="DarkGray" />
<GradientStop Offset="0.6" Color="White" />
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Style>

Answer 1

Instead of trying to change the Style in the Trigger, change the Background directly in it. Keeps you also from repeating the Style completely over again.
<Style x:Key="FancyButtonStyle" TargetType="{x:Type Button}">
<Setter Property="FontSize" Value="{Binding FontSize}"/>
<Setter Property="Foreground" Value="{Binding Foreground}"/>
<Setter Property="FontWeight" Value="{Binding FontWeight}"/>
<Setter Property="Width" Value="{Binding Width}"/>
<Setter Property="Height" Value="{Binding Height}"/>
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
<GradientStop Offset="0.0" Color="White" />
<GradientStop Offset="0.2" Color="DarkGray" />
<GradientStop Offset="0.6" Color="Black" />
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
<GradientStop Offset="0.0" Color="Black" />
<GradientStop Offset="0.4" Color="DarkGray" />
<GradientStop Offset="0.6" Color="White" />
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
I'm not sure if you even can change the Style of itself in the Trigger the way you intend to, because it is not set via a Setter Tag in the target control itself.