Is .nav a class or an element? If it's a class, as the leading . implies, you also have to look at the elements you apply it to, and make sure your css overrides those elements' properties. (Hint: If you are using !important you are doing it wrong.) So not just .nav but also p .nav li .nav p#superimportantheading .nav and so on depending on your specific circumstances.
General principle: if you ever need element1.class1 (for example "td.foobar")
to override element2.class2 element1 (for example "tr.widget td")
make sure you include, by name, element2.class2 element1.class1 ("tr.widget td.foobar")