[Twenty Twelve] Remove menu border

Hi

I’m trying to remove the top and bottom border on the Twenty twelve menu.
This is what I’ve got in the user.css

.main-navigation ul.nav-menu, .main-navigation div.nav-menu > ul { border-bottom: 0px; border-top: 0px;}

This works fint for everything but IE8 (and below probably). I know that I can edit the ie.css file in …/wp-content/themes/twentytwelve/css/ie.css but this way the changes will be lost if I update the theme.

Is there another way to do this?

MadsRH