You're welcome!
My usual take on CSS is "don't," but if you're sure about it, I would suggest making the text dark and the background light. If you don't like that, try light text on a dark background. What you definitely don't want to do is pick mid-range shades for both text and background, because people who don't have sharp eyes might not be able to read it at all that way.
What most people do here is leave the colors at default values, so it'll look fine to each reader regardless of what forum theme they're using. I use "Forever Standing," so I see most forum text as black on a very light blue, but here in this conversation I'm seeing black on a mid-gray so it's harder for me to read.