You probably need to be more specific with your rule selector (the 'a' part). If another rule is more specific, it will overwrite yours. Examples:
An 'a' tag with a class: a.someclass
An 'a' tag as the child of a 'div' tag: div > a
An 'a' tag after a tag with an id: #someid + a
If any of such rules exist, they will overwrite yours. You can see what rules are active in most browsers by right-clicking something and choosing 'Inspect'
Other things: Make sure the little checkbox 'Use Custom CSS' below is checked. Try restarting nbb and clearing your browser cache if nothing else works.