CSS to add space between menu elements

In my web you can see a menu composed by images, those images have transparent margins to simulate space. I’s a bad, rude and dirty solution and i want to do it the right way. (CSS)

I’ve found a method to edit margins between menu elements, but it affects to all the elements margins. The problem is that I need to set a bigger space for the last element, the login/register button.
What CSS code can I append to the theme to do that? (notice that i will replace the images when to apply the fix)

Alexandre.

The page I need help with: [log in to see the link]