ボタンのスタイリング中に、コンテンツ プレゼンターをラップしている境界線を確認します。PointOver Viewstate の境界線の色または太さのプロパティを変更してみてください。または、まったく必要ない場合は、次のようにすべての状態の境界線の太さを完全に削除できます。
<Style x:Key="ButtonStyle1" TargetType="Button">
<Setter Property="Background" Value="{StaticResource ButtonBackgroundThemeBrush}"/>
<Setter Property="Foreground" Value="{StaticResource ButtonForegroundThemeBrush}"/>
<Setter Property="BorderBrush" Value="{StaticResource ButtonBorderThemeBrush}"/>
<Setter Property="BorderThickness" Value="{StaticResource ButtonBorderThemeThickness}"/>
<Setter Property="Padding" Value="12,4,12,4"/>
<Setter Property="HorizontalAlignment" Value="Left"/>
<Setter Property="VerticalAlignment" Value="Center"/>
<Setter Property="FontFamily" Value="{StaticResource ContentControlThemeFontFamily}"/>
<Setter Property="FontWeight" Value="SemiBold"/>
<Setter Property="FontSize" Value="{StaticResource ControlContentThemeFontSize}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Border">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource ButtonPointerOverBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource ButtonPointerOverForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Border">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource ButtonPressedBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource ButtonPressedForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="Border">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource ButtonDisabledBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource ButtonDisabledForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused">
<Storyboard>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="FocusVisualWhite"/>
<DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="FocusVisualBlack"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Unfocused"/>
<VisualState x:Name="PointerFocused"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="Border" Background="{TemplateBinding Background}" Margin="3">
<ContentPresenter x:Name="ContentPresenter" ContentTemplate="{TemplateBinding ContentTemplate}" ContentTransitions="{TemplateBinding ContentTransitions}" Content="{TemplateBinding Content}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
<Rectangle x:Name="FocusVisualWhite" IsHitTestVisible="False" Opacity="0" StrokeDashOffset="1.5" StrokeEndLineCap="Square" Stroke="{StaticResource FocusVisualWhiteStrokeThemeBrush}" StrokeDashArray="1,1"/>
<Rectangle x:Name="FocusVisualBlack" IsHitTestVisible="False" Opacity="0" StrokeDashOffset="0.5" StrokeEndLineCap="Square" Stroke="{StaticResource FocusVisualBlackStrokeThemeBrush}" StrokeDashArray="1,1"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>