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.
We are building an insiders page using nodebb at collegemarching.com and we are looking for help updating our test site(insiders.collegemarching.com) to look professional and match our brand. Someone with nodebb implementation experience would be prefered. PM if you are interested, and we can discuss full scope and price quotes.
The CollegeMarching Team