Use the "Inspect" button. Click on the item you are interested in. On the right side, Firebug shows all the styles of the element and the line number + line from which they were obtained.
Find the style you are trying to apply and see if it has a touch. If so, then it is redefined by something higher in the hierarchy.
Now you need to decide how you want to solve it. Either by making your css entry more specific to the element in question, or by changing your styles in some other way.
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