public Style TimescaleRulerStyle { get; set; }

Public Property TimescaleRulerStyle As Style

<Window xmlns:dxgn="http://schemas.devexpress.com/winfx/2008/xaml/gantt"
        xmlns:dxgnn="http://schemas.devexpress.com/winfx/2008/xaml/gantt/internal">
    <dxgn:GanttControl ItemsSource="{Binding Tasks}">
    ...
        <dxgn:GanttControl.View>
            <dxgn:GanttView ...>
                <dxgn:GanttView.TimescaleRulerStyle>
                    <Style TargetType="{x:Type dxgnn:TimescaleRulerControl}">
                        <Setter Property="Background" Value="#FFFAB339"/>
                        <Setter Property="Foreground" Value="White"/>
                        <Setter Property="BorderThickness" Value="0"/>
                        <Setter Property="FontStyle" Value="Italic"/>
                        <Setter Property="TickBrush" Value="White"/>
                    </Style>
                </dxgn:GanttView.TimescaleRulerStyle>
            </dxgn:GanttView>
        </dxgn:GanttControl.View>
    </dxgn:GanttControl>
</Window>