CSS Styles to only One Class

CSS Styles to only One Class

Using Zurb's Foundation, I'm attempting to add my own styles to the
navbar, which is working great. Unfortunately, Zurb's responsive navbar
changes its look when the viewing container becomes smaller. When it does
this, it adds a second class to the nav element.
Before responsive change:
<nav class="top-bar">
After responsive change:
<nav class="top-bar expanded">
My custom styles look terrible for this expanded navbar, and I was
wondering if there was a way to only apply styles when an element
contained solely 1 class and not multiple. I know it's very easy to only
apply styles when an element has two or more classes, but can you exclude
one?
Thanks!