Magazine Basic Button shape change?

My web site is coming along nicely and I am wondering if I can change the shape of the buttons in the main menu area. I have been able to add pages as sub menu buttons which is very good but I cannot work out how to change the shape of the button.

Is this at all possible and if it is can someone explain very simply what I need to do to achieve this – I am thinking it might be nice to have a menu button with more rounded edges than just a straight forward rectangle.