I don't do this because I'm emo or depressed as some over-analyzers might think. I find the darker colors much more inviting and... enveloping, if you will. It's easier on the eyes than bright colors. A page that is really vibrant seems like an assault on the eyes, but a darker one invites you to keep looking because it isn't uncomfortable to look at.
Bright page themes seem really fake to me and dark ones seem more honest. Like the dark ones aren't relying on flashy things to get attention. It's confident in it's content.
I may have just outlined my views on life and people without even realizing it...
Perhaps I'm crazy.