I don't know if you have figured it out by now since your question was posted a while back. But I was struggling with the same thing.
What I did is that instead of having the navigation bar defined on each of the pages, I created a webblock that contains my Navigation bar with the necessary links.
The navigation bar has an input that contains the Section/Page that I am on, so I created a StaticEntity that will contain all the Sections from the NavigationBar.
Then on each link of the Navigation Bar I added an Extended Property of 'class' that will have a value of an empty string if it doesn't match the Section and "Active" if it does match.
Hopes this helps.
I am so sorry for the delay in this reply. This issue should be solved on the next version of Silk since you'll be able to set the active item as a parameter of the bottom bar.
Thanks Andres Moreno.
It worked as per your comment.