How to write style template to Popup control?

Question

I have a lot of popups in application (.NET Framework 4, WPF) and I have to set one style to all of them. Example popup looks like that:

<Popup PopupAnimation="Fade" MinWidth="600" MinHeight="200" Placement="Center" VerticalAlignment="Center" HorizontalAlignment="Center" IsEnabled="True" IsOpen="False">
    <Grid Width="Auto" Height="Auto" Background="Gray">
        <Grid.RowDefinitions>
            <RowDefinition Height="30"/>                   
            <RowDefinition Height="Auto"/>           
        </Grid.RowDefinitions>
        <Border BorderThickness="2" CornerRadius="8" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Grid.RowSpan="2">
            <Border.BorderBrush>
                <SolidColorBrush Color="Gray"/>
            </Border.BorderBrush>
            <Border.Background>
                <SolidColorBrush Color="White"/>
            </Border.Background>
        </Border>

        <StackPanel Grid.Row="0">
            <Label Foreground="Blue" Content="Popup_Title"/>
        </StackPanel>

        <GroupBox Grid.Row="1" Header="Popup example content">
            <StackPanel>                      
                   ...                           
            </StackPanel>
        </GroupBox>      
    </Grid>
</Popup>

How can I take styles like borders and background to a style template? I can't write Style with TargetType Popup and modify it's Property="Template" because Popup Control doesn't have Property="Template". So how can I write style to those Popups?

EDIT: Exact working style:

    <Style x:Key="PopupContentStyle" TargetType="ContentControl">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ContentControl">
                <Grid Width="Auto" Height="Auto" Background="Gray">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="30"/>
                        <RowDefinition Height="Auto"/>
                    </Grid.RowDefinitions>
                    <Border BorderThickness="2" CornerRadius="8" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Grid.RowSpan="2">
                        <Border.BorderBrush>
                            <SolidColorBrush Color="Gray"/>
                        </Border.BorderBrush>
                        <Border.Background>
                            <SolidColorBrush Color="White"/>
                        </Border.Background>
                    </Border>

                    <StackPanel Grid.Row="0">
                        <Label Foreground="Blue" Content="Popup_Title"/>
                    </StackPanel>

                    <GroupBox Grid.Row="1" Header="Popup example content">
                        <StackPanel>
                            <ContentPresenter />
                        </StackPanel>
                    </GroupBox>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Answer 1

I'd recommend wrapping your Popup's Content in something like a ContentControl or a HeaderedContentControl and setting the style of that

<Popup>
    <ContentControl Style="{StaticResource PopupContentStyle}">
        ...
    </ContentControl>
</Popup>

Example style...

<Style x:Key="PopupContentStyle" TargetType="{x:Type ContentControl}">
    <Setter Property="ContentTemplate">
        <Setter.Value>
            <DataTemplate>

                <Grid Width="Auto" Height="Auto" Background="Gray">
                   <Grid.RowDefinitions>
                        <RowDefinition Height="30"/>                   
                        <RowDefinition Height="Auto"/>           
                    </Grid.RowDefinitions>
                    <Border BorderThickness="2" CornerRadius="8" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Grid.RowSpan="2">
                        <Border.BorderBrush>
                            <SolidColorBrush Color="Gray"/>
                        </Border.BorderBrush>
                        <Border.Background>
                            <SolidColorBrush Color="White"/>
                        </Border.Background>
                    </Border>

                    <StackPanel Grid.Row="0">
                        <Label Foreground="Blue" Content="Popup_Title"/>
                    </StackPanel>

                    <GroupBox Grid.Row="1" Header="Popup example content">
                        <StackPanel>                      
                               <ContentPresenter />                         
                        </StackPanel>
                   </GroupBox>      
                </Grid>
            </DataTemplate>
        </Setter.Value>
    </Setter>
</Style>