How to change my Group-box style - wpf

Question

So this is my GroupBox:
<GroupBox
Header="My header"
Background="Transparent"
Foreground="Gainsboro"
BorderBrush="Gainsboro" />
And i have several things that i want to change/issues:
The Foreground is still in black although i have changed it.
My boder around the Header: i want to remove it in order to achieve this kind of style (the header is over the border):
All header is with upper case.
Edit:
After add the style that user #mm8 suggested, this is the results:
Why i can see duplicate border ?

Answer 1

You could override the template for the GroupBox:
<GroupBox Header="My header" Style="{x:Null}">
<GroupBox.Template>
<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 BorderBrush="Transparent" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="4" Grid.Column="0" CornerRadius="4" Grid.Row="1" Grid.RowSpan="3"/>
<Border BorderBrush="White" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="4" CornerRadius="4" Grid.Row="1" Grid.RowSpan="3">
<Border.OpacityMask>
<MultiBinding ConverterParameter="7">
<MultiBinding.Converter>
<BorderGapMaskConverter />
</MultiBinding.Converter>
<Binding ElementName="Header" Path="ActualWidth"/>
<Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}"/>
<Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}"/>
</MultiBinding>
</Border.OpacityMask>
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="3">
<Border BorderBrush="White" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="2"/>
</Border>
</Border>
<Border x:Name="Header" Grid.Column="1" Padding="3,1,3,0" Grid.Row="0" Grid.RowSpan="2" Background="LightGreen" BorderBrush="Black" CornerRadius="2" BorderThickness="1">
<ContentPresenter ContentSource="Header" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Border>
<ContentPresenter Grid.ColumnSpan="2" Grid.Column="1" Margin="{TemplateBinding Padding}" Grid.Row="2" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
</Grid>
</ControlTemplate>
</GroupBox.Template>
<TextBlock>content...</TextBlock>
</GroupBox>

Answer 2

Create your own set of controls to have full control, similar to this:
<Grid Width="100" Height="200" Background="DodgerBlue">
<Grid.RowDefinitions>
<RowDefinition Height="20"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Border BorderThickness="1" BorderBrush="Gainsboro">
<TextBlock Foreground="Gainsboro">Header</TextBlock>
</Border>
<Border Grid.Row="1" BorderThickness="1" BorderBrush="Gainsboro">
<!--Content-->
</Border>
</Grid>