I have a very similar issue, however, my parent menu uses icons, so using the OP as an example,
If Banquette is highlighted it would be:
.sidebar .menu .banquette a { background: url(images/banquette.png) no-repeat; }
.sidebar .menu .banquette a:hover,
.sidebar .menu .banquette a:focus,
.sidebar .menu .banquette a.active { background: url(images/banquette_selected.png) no-repeat; }
How would I set the sub navigation (designer, traditional) to capture that behavior?