Reduce space inside border

I’m setting up a wp site at blog.ridwan.me.

I have border lines above and below the main menu. I want to make these lines narrower, so the menu links have less white between them and the lines.

The css style for this right now is
#main-navigation {
border: solid #333;
border-width: 1px 0px 3px;
}

I tried using border-padding, and this works to extend the space but not to reduce it (0px is still too much space, and negative padding is not allowed).

Any ideas on what I can do?