Board Thread:Support Requests - Designing Your Wikia/@comment-3135378-20151205215003/@comment-4731443-20151206004102

You can remove the pink line with: nav li.marked { border-top: unset; }

As I said above, I don't think there's a way you could change the bar's color depending on the active tab using only CSS. It's a separate element that doesn't change based on the selected tab.