You are right that the TimeRulerLineStyle should be customized, however, in order to distinguish the needed lines, you should create a custom TimeRulerItemStyleSelector
and add some additional logic in SelectStyle method. I have attached a sample project which demonstrates the approach, please download it and give it a try. Note that I've also customized the MajorHorizontalTimeRulerItemStyle in order to style the ticks in the TimeRuler as well.
I hope this is helpful.