Changing Navigation Links color

Do not edit the Twenty Twelve theme. It is the default WordPress theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. First create a child theme for your changes. Or install a custom CSS plugin.

Hey there,
After you have created the child theme or installed a custom CSS plugin as esmi nicely said you can include this code into the child themes style.css or into the custom css plugin.

.main-navigation ul.nav-menu, .main-navigation div.nav-menu > ul {
  background-color: #FFFFFF;
  border-bottom: 1px solid #EDEDED;
  border-top: 1px solid #EDEDED;
  display: inline-block !important;
  width: 100%;
}