Just had to do this myself and found your post while searching. You need to add RepeatButtons for ScrollBar.LineUpCommand and LineDownCommand buttons (above and below the track) to your ScrollBar ControlTemplate as follows:
<ControlTemplate TargetType="{x:Type ScrollBar}">
<Grid SnapsToDevicePixels="true">
<Grid.RowDefinitions>
<RowDefinition MaxHeight="18"/>
<RowDefinition Height="0.00001*"/>
<RowDefinition MaxHeight="18"/>
</Grid.RowDefinitions>
<RepeatButton Grid.Row="0" Height="18" Command="{x:Static ScrollBar.LineUpCommand}" />
<Track x:Name="PART_Track" Grid.Row="1" IsDirectionReversed="true" IsEnabled="{TemplateBinding IsMouseOver}">
<Track.DecreaseRepeatButton>
<RepeatButton Command="{x:Static ScrollBar.PageUpCommand}" Style="..."/>
</Track.DecreaseRepeatButton>
<Track.IncreaseRepeatButton>
<RepeatButton Command="{x:Static ScrollBar.PageDownCommand}" Style="..."/>
</Track.IncreaseRepeatButton>
<Track.Thumb>
<Thumb Style="..." Width="14"/>
</Track.Thumb>
</Track>
<RepeatButton Grid.Row="2" Height="18" Command="{x:Static ScrollBar.LineDownCommand}" />
</Grid>
Sacha has a full sample here link.