When I open a lot of contact sheets, it is not easy to see fastly which is actived because there is not a great difference between ligtht grey and dark grey in the top.
Best regards from France !
You didn't state your OS platform, but from your description I think you're referring to the Mac OS X version. There are several visual cues that indicate which tab is selected. First, there are shadows cast from the selected tab onto surrounding tabs. Second, the selected tab has no division above it and blends into the title bar of the window. Third, the title of the window itself changes to the name of the selected tab.
Perhaps you're not noticing these cues, but I have never had any issue identifying which tab is active, even when working rapidly.
-Kirk