Navigation bar shading tweak – based on WP Twenty-Eleven theme

Hi, I want to find a way to slightly change the rollover appearance on the navigation bar of this site:

https://bit.ly/TN1iym

As you’ll see, when you roll over a link in the nav bar, the lighter green box appears to highlight this – that’s fine. But I want this rollover to not be a solid square block, but the have some of the depth and shading of the background (i.e. non-highlighted) elements of the nav bar.

This site is based on the WP Twenty Twelve theme.
Any ideas on how best to achieve this? Thanks in advance!