GroupBoxStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

      <Style x:Key="MyGbStyle" TargetType="GroupBox">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="GroupBox">
                        <Grid>
                            <Grid.RowDefinitions>
                                <RowDefinition Height="Auto" />
                                <RowDefinition Height="*" />
                            </Grid.RowDefinitions>
                            <Border Grid.Row="0" BorderThickness="1"
                                                      BorderBrush="#13a2e8"
                                                      Background="#13a2e8">
                                <Label Foreground="White">
                                    <ContentPresenter Margin="0" ContentSource="Header" RecognizesAccessKey="True" />
                                </Label>
                            </Border>
                            <Border Grid.Row="1" BorderThickness="1,0,1,1" BorderBrush="#13a2e8">
                                <ContentPresenter Margin="4" />
                            </Border>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
</ResourceDictionary>