<style>
div {
    color: red !important;
}
div#azul {
    color: blue;
}
</style>
<div id="azul">Este texto es rojo</div>

<div class="skin-rob">
    <div class="navbar navbar-default">
        <div class="navbar-header">
            <a class="navbar-brand" href="#">Bootstrap 3</a>
        </div>
    </div>
    ...
</div>

.skin-rob .navbar-default .navbar-brand, .navbar-default {
    background-color: blue;
}

.skin-rob .navbar-default .navbar-brand:hover, .navbar-default .navbar-brand:focus {
    background-color: red;
}