How can I add a border to main navigation?

Add this to the bottom of customise.css:

navbar-inner {
border: 1px solid #000000;
}

That will add a white border.  Change "#00000" to the HEX Colour code you want the border to be.  Here are some hex codes: http://html-color-codes.info/

You can change "1px" to "2px" to make it thicker.