I've come to realize that Doomworld is far and away the most soothing on the eyes. I use the default theme and the navy blues/blacks make me far more likely to zone out staring at dwf than any other site that I check out regularly.
Pages with dark but muted colors and light text are very easy on the eyes. I always tend to go with dark themes if I can at any of the message boards I visit. It really strains your eyes staring at a bright screen for hours on end.