The best reference is JFreeChart's API/Javadoc; for instance, the ValueMarker class is defined here:
http://www.jfree.org/jfreechart/api/javadoc/org/jfree/chart/plot/ValueMarker.html
Line weight and color can be set by using one of the overloaded constructors;
ValueMarker(double value, Paint paint, Stroke stroke)
Paint is a base Java class that's implemented by Color, so you can just use system.gui.color(r, g, b).
Stroke is easiest to define with the BasicStroke class - so just import that class, and use a new instance in the call when you create a new ValueMarker instance.