When Firebug says "element.style", that means that the style is defined in-line, on the tag itself, using a style attribute.  To override this, you will have to either change the container's markup itself, or you can add !important to the end of your overriding style.

For example, your style might looks like this:

.header-class { color: #000; background-color: #0f0 !important; }

Hope that helps,