Looks good! A few suggestions:
- I agree with the others who said the header is too big. I think the height could be reduced by half. The height of the menu bars could be reduced slightly as well.
- For the black background I'd use the official color RGB 51,51,51 rather than solid black.
- For the inset gray background (behind the posts), I'd use a slightly darker shade for more contrast with the white boxes to help define and separate them from each other,