If you are just asking how to get an underline in the header area of a tab item then you might set the TabHeaderTemplate to a DataTemplate that has a TextBlock and sets its TextDecorations to Underline. e.g.

<igDock:ContentPane Header="Forums">
  <igDock:ContentPane.TabHeaderTemplate>
    <DataTemplate>
      <TextBlock x:Name="tb" Text="{Binding}" />
      <DataTemplate.Triggers>
        <DataTrigger Binding="{Binding Path=IsSelected, RelativeSource={RelativeSource AncestorType={x:Type igDock:PaneTabItem}}}" Value="True">
          <Setter Property="TextDecorations" TargetName="tb" Value="Underline" />
        </DataTrigger>
      </DataTemplate.Triggers>
    </DataTemplate>
  </igDock:ContentPane.TabHeaderTemplate>
</igDock:ContentPane>

If you want the tab area and tab items to look like those forums then you would need to create your own custom ControlTemplate for the TabGroupPane and PaneTabItem. You can look at our default xaml (included in the DefaultStyles directory) as a starting point for your custom styles. If you are specifically trying to change the look of the tab control/tab items within the document area then look for the styles in the default xaml that have an x:Key of igDock:TabGroupPane.DocumentTabGroupTemplateKey and igDock:PaneTabItem.DocumentTabItemTemplateKey respectively.