Переопределение темы ButtonChrome будет проще.
Создайте шаблон и удалите RenderMouseOver = "{TemplateBinding IsMouseOver}"
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Microsoft_Windows_Themes:ButtonChrome x:Name="Chrome" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" RenderMouseOver="{TemplateBinding IsMouseOver}"RenderPressed="{TemplateBinding IsPressed}" RenderDefaulted="{TemplateBinding Button.IsDefaulted}" SnapsToDevicePixels="True">
<ContentPresenter ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" ContentStringFormat="{TemplateBinding ContentStringFormat}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Microsoft_Windows_Themes:ButtonChrome>
<ControlTemplate.Triggers>
<Trigger Property="IsKeyboardFocused" Value="True">
<Setter Property="RenderDefaulted" TargetName="Chrome" Value="True"/>
</Trigger>
<Trigger Property="ToggleButton.IsChecked" Value="True">
<Setter Property="RenderPressed" TargetName="Chrome" Value="True"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Foreground" Value="#FFADADAD"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
А затем добавьте свою собственную обработку мыши
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FFF3E8D5" Offset="0"/>
<GradientStop Color="#FFF49B03" Offset="1"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
Это должно решить это! :)