I really do like the Czar theme, and I'm not just saying that because it's what I'm used to. It's a really readable board style. If you like, I can fiddle with the images and .css to recreate the Czar Green theme out of the Czar Orange. Or hell, I could even do Czar Blue or Czar Charcoal or Czar Periwinkle if anyone wants one of those (not Czar Periwinkle).
But as far as existing themes go, I like these two:
Because they're clean and readable. After some thought, I'm not really digging the color highlighting on the posts with the current theme, and the icons are rather ambiguous and confusing. I mean, what does this mean: <img src="http://www.scirra.com/phpBB3/styles/saphic/imageset/topic_read_hot_mine.gif">. Haven't got a clue, but it sure is big and blinky.