Black background + dark text = waste of time
Have you ever printed black ink on dark paper? How about bright yellow on neon green? Probably not. Why? It sounds ridiculous since the whole point is to read what you have.
Despite how obvious this seems when talking about printing on paper, it's amazing the number of people who told us about PowerPoint presentations with text that didn't have enough contrast to be readable. If it's not easily readable, then it's a waste of time.
Make sure, if you use dark text, use a light background -- and vice versa.