2
Well there are usually two methods...
One involving the template for the menu itself and based on styles.
And you can also just literally place your new menu in the index page using the style rules that are used in the menu...or new ones. Basically just links with style rules defined in your css.
If you examine the K2 theme for example you will notice a little table on the index (theme.html) page of that set. That table will most likely have links with a style defined on them. ie: "a class "whatever"" or div class....or td id... etc
Once you peg those out find the coreresponding style rules in the css style sheet. You can then mess around with it some more.
Well This was a bit of a run on sentance, sorry. But I hope it gets you started in the right direction.