There is of course a number of ways you can add a highlight to the selected menu node. Since I have no way of viewing your custom control I can recommend you the neat RadMenu samples we have available here. Have a good look at all the information there.
I hope this is helpful. Feel free to contact us if you need additional assistance.
From what I understand you are using our old navigation menu. The easiest way for you to get the page highlated is to use your template. Select Custom Navigation. Under Design Settings set path to a template where you have Navigation control - say RadPanelBar. Get the current node in the code behind and apply some class.
My apologies for the unwanted effects of our navigation control. I do understand that working with so much HTML can prove troublesome, but it is required in order to sustain the functionality we provide. Sitefinity's navigation is an ASP.NET control and as such it generates and repeats a lot of markup in order to execute its designed behavior. More information about ASP.NET user controls can be found in MSDN.Regards,