You are having the exact same problem I had. That little margin appeared between my logo and my navigation section at the top of my page. At first I was not sure how to fix it nor did I have the time to try and fix it myself. So I contacted the people who originally created the skin and they fixed it for me.

Trouble is I am not to sure what they did to fix the issue. But I can tell you that it had something to do with the code in the tpl_main_page.php and or the stylesheet.css.

Sorry I cannot help any further. But changes to either one or both of those files fixed the problem for me.