Возможно, фон вашего элемента управления установлен на {x:Null}
по умолчанию, попробуйте установить <Setter Property="Background" Value="Transparent"/>
{x: Null не обнаружит мышь, прозрачный будет.
Редактировать (дополнение):
OverridesDefaultStyle = Истина должна быть в style.setters, а не в триггере.
Пример:
<ToggleButton Content="ToggleButton" Height="30" Width="110" >
<ToggleButton.Style>
<Style TargetType="ToggleButton">
<Setter Property="SnapsToDevicePixels" Value="True"/>
<Setter Property="UseLayoutRounding" Value="True"/>
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Offset="0" Color="#FFFFFF"/>
<GradientStop Offset="0.48" Color="#DDDDDD"/>
<GradientStop Offset="0.5" Color="#CCCCCC"/>
<GradientStop Offset="0.52" Color="#BBBBBB"/>
<GradientStop Offset="1" Color="#CCCCCC"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush">
<Setter.Value>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Offset="0" Color="#DDDDDD"/>
<GradientStop Offset="0.48" Color="#BBBBBB"/>
<GradientStop Offset="0.5" Color="#AAAAAA"/>
<GradientStop Offset="0.52" Color="#999999"/>
<GradientStop Offset="1" Color="#AAAAAA"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToggleButton">
<Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}"
CornerRadius="2" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" UseLayoutRounding="{TemplateBinding UseLayoutRounding}">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
<ControlTemplate.Triggers>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsMouseOver" Value="True"/>
<Condition Property="IsChecked" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Offset="0" Color="#FFFFFF"/>
<GradientStop Offset="0.48" Color="#EEEEEE"/>
<GradientStop Offset="0.5" Color="#DDDDDD"/>
<GradientStop Offset="0.52" Color="#CCCCCC"/>
<GradientStop Offset="1" Color="#DDDDDD"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsMouseOver" Value="True"/>
<Condition Property="IsChecked" Value="True"/>
</MultiTrigger.Conditions>
<Setter Property="BorderBrush" Value="#666666"/>
<Setter Property="BorderThickness" Value="2,2,1,1"/>
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Offset="0" Color="#AAAAAA"/>
<GradientStop Offset="0.48" Color="#999999"/>
<GradientStop Offset="0.5" Color="#AAAAAA"/>
<GradientStop Offset="0.52" Color="#BBBBBB"/>
<GradientStop Offset="1" Color="#DDDDDD"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsChecked" Value="True"/>
<Condition Property="IsMouseOver" Value="False"/>
</MultiTrigger.Conditions>
<Setter Property="BorderBrush" Value="#333333"/>
<Setter Property="BorderThickness" Value="2,2,1,1"/>
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<GradientStop Offset="0" Color="#999999"/>
<GradientStop Offset="0.48" Color="#888888"/>
<GradientStop Offset="0.5" Color="#999999"/>
<GradientStop Offset="0.52" Color="#AAAAAA"/>
<GradientStop Offset="1" Color="#CCCCCC"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ToggleButton.Style>
</ToggleButton>
Что касается свечения:
Полностью замените ContentPresenter следующим:
<Grid>
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
<ContentPresenter x:Name="ContentPart" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
TextBlock.Foreground="#5588FF" Visibility="Collapsed">
<ContentPresenter.Effect>
<BlurEffect />
</ContentPresenter.Effect>
</ContentPresenter>
</Grid>
и добавьте этот простой триггер:
<Trigger Property="IsChecked" Value="True">
<Setter TargetName="ContentPart" Property="Visibility" Value="Visible"/>
</Trigger>