Usability: Highlight the navigation item of what the user is currently viewing

Highlight what reousrce/activity is currently viewed by the user on the navigation block item list. So the user could visualize where he/she is at, relative to the course activity flow/list.

The .current_branch was available but not used.
You might consider a different visual effect.

(Not sure if this is the right css file this fix should go into)

Nadav :-)