If you don't want to change the header graphic, I would go with a black/dark gray background or maybe white if you can add a gray border to the content area.
However, I think the design needs some more personality, if you know what I mean. Give your users a bit of visual reference that will help them remember your blog. Keeping focus on content is good, but that doesn't mean you have to stick with no visual reference whatsoever. A well designed logo could do the trick, even.
I also share the opinion that the colors you used in the top image aren't particularly attractive, but that's just a matter of taste. If you really want to use those colors, though, try a top-bottom gradient instead.
Let me know if I can help you any further, I'm a designer and I tweak themes for clients and friends all the time. I just recently tweaked
Bluetoothlounge.com - Bluetoooth source for a friend (designed the top banner and did other small changes). Sometimes small changes make a huge difference.
I hope this helps.