Кнопка со странным фоном закончилась?

Я использую это Button:

<Button Name = "buttonAttiva"
        HorizontalAlignment = "Center"
        VerticalAlignment = "Top"
        Margin = "0,560,0,0">
    <TextBlock Text = "Attiva" />

Это стиль:

<Style TargetType = "{x:Type Button}"
       BasedOn = "{StaticResource MahApps.Styles.Button}">
    <Setter Property = "HorizontalAlignment"
            Value = "Left" />
    <Setter Property = "VerticalAlignment"
            Value = "Top" />
    <Setter Property = "Width"
            Value = "150" />
    <Setter Property = "Height"
            Value = "46" />
    <Setter Property = "BorderThickness"
            Value = "0" />
    <Setter Property = "BorderBrush"
            Value = "Transparent" />
    <Setter Property = "Background"
            Value = "#81b3d4" />
    <Setter Property = "Foreground"
            Value = "#ffffff" />
    <Setter Property = "FontWeight"
            Value = "Bold" />
    <Setter Property = "FontSize"
            Value = "18" />
    <Setter Property = "VerticalContentAlignment"
            Value = "Center" />
    <Setter Property = "FontWeight"
            Value = "Bold" />
    <Setter Property = "FocusVisualStyle"
            Value = "{x:Null}" />
    <Setter Property = "controls:ControlsHelper.MouseOverBorderBrush"
            Value = "#000000" />
    <Setter Property = "controls:ControlsHelper.FocusBorderBrush"
            Value = "#000000" />
    <Setter Property = "controls:ControlsHelper.FocusBorderThickness"
            Value = "0" />
    <Style.Resources>
        <SolidColorBrush x:Key = "MahApps.Brushes.Gray7"
                         Color = "#000000" />
        <SolidColorBrush x:Key = "MahApps.Brushes.Gray8"
                         Color = "#000000" />
    </Style.Resources>
    <Style.Triggers>
        <Trigger Property = "IsFocused"
                 Value = "True">
            <Setter Property = "Background"
                    Value = "#000000" />
            <Setter Property = "BorderThickness"
                    Value = "0" />
            <Setter Property = "BorderBrush"
                    Value = "#000000" />
        </Trigger>
        <Trigger Property = "IsMouseOver"
                 Value = "True">
            <Setter Property = "Background"
                    Value = "#000000" />
            <Setter Property = "BorderThickness"
                    Value = "0" />
            <Setter Property = "BorderBrush"
                    Value = "#000000" />
        </Trigger>
        <Trigger Property = "IsPressed"
                 Value = "True">
            <Setter Property = "Background"
                    Value = "#000000" />
            <Setter Property = "BorderThickness"
                    Value = "0" />
            <Setter Property = "BorderBrush"
                    Value = "#000000" />
        </Trigger>
    </Style.Triggers>
</Style>

Что правильно отображается как:

Но когда я навожу мышь, появляется странный «серый» фон:

Как установить правильный оранжевый фон?

Обратите внимание: я параллельно использую MahApps.Metro, который, я думаю, может переопределить некоторые свойства?