I've been looking at the code of BlogEngine.NET and playing around with it a bit lately, so I decided to create a theme to see how easy it is. And, it is actually really simple to do.
I actually used the Cogitation theme as the basis and just morphed it into the below theme I call SimpleBlack. If you would like to use it, go ahead.
Download: BlogEngine.NET v1.2 SimpleBlack Theme