I like both if they're designed well. Light for during the day, and when the sun is setting or it's dark, a dark theme so it doesn't cause my retinas to melt and drip out of my eyelids.
For me it really depends on the kind of forum I am visiting. If I am on a general forum then I prefer to see light colours used but for forums such as gaming forums it's the norm for me to see darker colours used on these kind of forums and I feel it suits them better.