I faced the issue.

Thankfully, we can override the style.

            <Style
                x:Key="CustomAxisLineStyle"
                TargetType="Line">
                <Setter Property="Stroke" Value="Red" />
                <Setter Property="StrokeThickness" Value="2" />
            </Style>

        <chart:RadCartesianChart.VerticalAxis>
            <chart:LinearAxis LineStyle="{StaticResource CustomAxisLineStyle}" />
        </chart:RadCartesianChart.VerticalAxis>