Background of inactive Primary Menu Tabs

I have been working on trying to customise the acquia marina theme. I am trying to change the background colour of the primary menu. I have managed to do this successfully but the grey menu tabs still have white at the corners. I removed this for the active tab by changing the image file. I believe the other tabs achieve this via CSS but I cannot find, despite much trial and error, where to make this change.

Any help would be much appreciated.

Thanks
Duncan