Because Mish Shedlock told me to. And he has a lot of readers. I thought black background and white letters was cool, but Mish differs, and when Mish differs on the nuts and bolts of blogging, I defer.
I may experiment with a few other “themes” but for now I like this one. Mainly I like to have the blogroll prominent; make it clear and easy for comments; and a look that isn’t too busy or gimmicky. The above book shelf thing is as gimmicky as I can stand, really; but the other features override whatever objections I might have to the layout.
But that’s just me and my preferences. Feedback is welcome as always.