Header Right Menu, how to change the seize of the "background box"

Hallo! In the Header Right Menu I use small flags to navigate to WebPages in different languages. If a page is selected, the background colour behind the selected flag (page) changes, so the visitor knows better where he is. The flag is now visible in a "box". I would like to increase the height of this box, so I can use a little bit bigger flag in the menu. Lets say 5 pixels more height. How can this be done? Thanks!