resources/xaml/styles/ScrollBarStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
     
    <!-- Modern Thin ScrollBar Style -->
<Style x:Key="ModernScrollBar" TargetType="{x:Type ScrollBar}">
        <Setter Property="Width" Value="8"/>
        <Setter Property="MinWidth" Value="8"/>
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ScrollBar}">
                    <Grid>
                        <Track x:Name="PART_Track">
                            <Track.Thumb>
                                <Thumb>
                                    <Thumb.Template>
                                        <ControlTemplate TargetType="{x:Type Thumb}">
                                            <Border x:Name="ThumbVisual"
                                                    Background="#60808080"
                                                    CornerRadius="4"
                                                    Margin="1"/>
                                            <ControlTemplate.Triggers>
                                                <Trigger Property="IsMouseOver" Value="True">
                                                    <Setter TargetName="ThumbVisual" Property="Background" Value="#A0808080"/>
                                                </Trigger>
                                                <Trigger Property="IsDragging" Value="True">
                                                    <Setter TargetName="ThumbVisual" Property="Background" Value="#C0808080"/>
                                                </Trigger>
                                            </ControlTemplate.Triggers>
                                        </ControlTemplate>
                                    </Thumb.Template>
                                </Thumb>
                            </Track.Thumb>
                        </Track>
                    </Grid>
                     
                    <ControlTemplate.Triggers>
                        <Trigger Property="Orientation" Value="Vertical">
                            <Setter TargetName="PART_Track" Property="IsDirectionReversed" Value="True"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                     
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <Trigger Property="Orientation" Value="Horizontal">
                <Setter Property="Height" Value="8"/>
                <Setter Property="MinHeight" Value="8"/>
                <Setter Property="Width" Value="Auto"/>
            </Trigger>
        </Style.Triggers>
    </Style>
     
    <!-- Apply ModernScrollBar as default ScrollBar style -->
    <Style TargetType="{x:Type ScrollBar}" BasedOn="{StaticResource ModernScrollBar}"/>
     
</ResourceDictionary>