Is there a way to change the color and thickness of the vertical/horizontal default lines on a LineChart
Yes, you can change the default MajorGridLines of the axis as follows:
<igCA:XamChart.Axes>
<igCA:Axis AxisType="PrimaryX">
<igCA:Axis.MajorGridline>
<igCA:Mark Stroke="Red" StrokeThickness="2"/>
</igCA:Axis.MajorGridline>
</igCA:Axis>
<igCA:Axis AxisType="PrimaryY">
<igCA:Mark Stroke="Blue" StrokeThickness="2"/>
</igCA:XamChart.Axes>