Question

How do I remove the white borders?

Answer 1

You could set BorderThickness to 0, but that would also hide the black border. The white color is part of the ControlTemplate, so you’ll need to create a new template. Here is one I made by running the Reflector BamlViewer add-in on PresentationFramework.Aero and removing the two white borders:

<BorderGapMaskConverter x:Key="BorderGapMaskConverter"/>
<Style TargetType="{x:Type GroupBox}">
    <Setter Property="BorderBrush" Value="#D5DFE5" />
    <Setter Property="BorderThickness" Value="1" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type GroupBox}">
                <Grid SnapsToDevicePixels="true">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="6" />
                        <ColumnDefinition Width="Auto" />
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="6" />
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto" />
                        <RowDefinition Height="Auto" />
                        <RowDefinition Height="*" />
                        <RowDefinition Height="6" />
                    </Grid.RowDefinitions>
                    <Border CornerRadius="4" Grid.Row="1" Grid.RowSpan="3" Grid.Column="0" Grid.ColumnSpan="4" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="Transparent" Background="{TemplateBinding Background}" />
                    <Border Name="Header" Padding="3,1,3,0" Grid.Row="0" Grid.RowSpan="2" Grid.Column="1">
                        <ContentPresenter ContentSource="Header" RecognizesAccessKey="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                    </Border>
                    <ContentPresenter Grid.Row="2" Grid.Column="1" Grid.ColumnSpan="2" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                    <Border Grid.Row="1" Grid.RowSpan="3" Grid.ColumnSpan="4"  BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="3">
                        <Border.OpacityMask>
                            <MultiBinding Converter="{StaticResource BorderGapMaskConverter}" ConverterParameter="7">
                                <Binding ElementName="Header" Path="ActualWidth" />
                                <Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}" />
                                <Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}" />
                            </MultiBinding>
                        </Border.OpacityMask>
                    </Border>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>