Установить стиль большого пальца:
<Style x:Key="SliderThumbStyle" TargetType="{x:Type Thumb}">
<Setter Property="SnapsToDevicePixels" Value="true"/>
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<!--<Ellipse
Name="Ellipse"
Fill="Yellow"
Stroke="Yellow"
Height="10"
Width="{Binding Path=ThumbWidth, RelativeSource={RelativeSource TemplatedParent}}"
StrokeThickness="1" />-->
<Rectangle
Fill="Azure"
Stroke="Azure"
Height="7"
Width="{Binding Path=ThumbWidth, RelativeSource={RelativeSource TemplatedParent}}"
StrokeThickness="1"
Margin="0.1,.1,.1,.1"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Затем используйте этот стиль пользовательского элемента управления слайдером
<Style TargetType="{x:Type local:NvSliderControl}">
<Setter Property="Orientation" Value="Vertical" />
<Setter Property="Height" Value="50"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:NvSliderControl}">
<Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
<Grid>
<Track x:Name="PART_Track" >
<Track.Thumb>
<Thumb Style="{StaticResource SliderThumbStyle}">
</Thumb>
</Track.Thumb>
</Track>
</Grid>
</Border>
<ControlTemplate.Triggers>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>