I used to have a Navbar Guide published but it is not up-to-date and has been removed.
Try this code but it will really depend on what you have already so will need adapting.
.navbar .nav > li > a {
border: 1px solid yellow; /* Add a border */
border-radius: 0px 0px 0px 0px; /* No Rounded corners */
color: white; /* White text */
background-color: black; /* Change black */
background-image: linear-gradient(to bottom, grey, black); /* Change grey, black */
background-image: -webkit-linear-gradient (top, grey, black); /* Change grey, black */
background-repeat: repeat-x; /* Add shading */
}
.navbar .nav > li > a:hover {
border: 1px solid yellow; /* Add a border */
border-radius: 0px 0px 0px 0px; /* No Rounded corners */
color: red; /* Red Text on hover */
background-color: black; /* Change black */
background-image: linear-gradient(to bottom, grey, black); /* Change grey, black */
background-image: -webkit-linear-gradient (top, grey, black); /* Change grey, black */
background-repeat: repeat-x; /* Add shading */
}
/* Shade the current selected menu item */
.navbar .nav > li.current-menu-item > a, .navbar .nav > li.current-menu-ancestor > a {
color: red; /* Change Text red */
background-color: black; /* Change Bgrd black */
background-image: linear-gradient(to bottom, black, black); /* Change black, black */
background-image: -webkit-linear-gradient (top, black, black); /* Change black, black */
}