How to write style template to Popup control?

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>