DarkThemedSlider

<Style x:Key="SliderRepeatButton1" TargetType="RepeatButton">
    <Setter Property="SnapsToDevicePixels" Value="true" />
    <Setter Property="OverridesDefaultStyle" Value="true" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="RepeatButton">
                <Border SnapsToDevicePixels="True" 
                        Background="{StaticResource SMutedWhiteBackground}" 
                        Height="20"
                        CornerRadius="10"
                        Margin="0 0 -15 0"/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<Style x:Key="SliderRepeatButton" TargetType="RepeatButton">
    <Setter Property="SnapsToDevicePixels" Value="true" />
    <Setter Property="OverridesDefaultStyle" Value="true" />
    <Setter Property="IsTabStop" Value="false" />
    <Setter Property="Focusable" Value="false" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="RepeatButton">
                <Border SnapsToDevicePixels="True"
                        Background="{StaticResource SMutedGray}" 
                        Height="20"
                        CornerRadius="0 10 10 0"
                        Margin="-5 0 0 0"/>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<Style x:Key="SliderThumb" TargetType="Thumb">
    <Setter Property="SnapsToDevicePixels" Value="true" />
    <Setter Property="OverridesDefaultStyle" Value="true" />
    <Setter Property="Cursor" Value="Hand"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="Thumb">
                <Ellipse x:Name="Ellipse" Height="20" Width="20" 
                         StrokeThickness="3" 
                         Stroke="{StaticResource SMutedWhiteBackground}"
                         Fill="{StaticResource SMutedGray}"
                         Margin="-4"/>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsMouseOver" Value="true">
                        <Setter TargetName="Ellipse" Property="StrokeThickness"
                                Value="4"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<ControlTemplate x:Key="Slider" TargetType="Slider">
    <Grid Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" MinHeight="{TemplateBinding MinHeight}" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Track Grid.Row="1" x:Name="PART_Track" >
            <Track.IncreaseRepeatButton>
                <RepeatButton Style="{StaticResource SliderRepeatButton}" Command="Slider.IncreaseLarge" />
            </Track.IncreaseRepeatButton>
            <Track.DecreaseRepeatButton>
                <RepeatButton Style="{StaticResource SliderRepeatButton1}"  Command="Slider.DecreaseLarge" />
            </Track.DecreaseRepeatButton>
            <Track.Thumb>
                <Thumb Style="{StaticResource SliderThumb}"  />
            </Track.Thumb>
        </Track>
    </Grid>
</ControlTemplate>

<Style x:Key="Horizontal_Slider" TargetType="Slider">
    <Setter Property="Focusable" Value="False"/>
    <Setter Property="SnapsToDevicePixels" Value="true" />
    <Setter Property="OverridesDefaultStyle" Value="true" />
    <Style.Triggers>
        <Trigger Property="Orientation" Value="Horizontal">
            <Setter Property="MinHeight" Value="20" />
            <Setter Property="MinWidth" Value="104" />
            <Setter Property="Template" Value="{StaticResource Slider}" />
        </Trigger>
    </Style.Triggers>
</Style>