Your on the right track by using margin left/right auto and using a doctype... it's trying to center your #content but the 1800px width is just too wide.
Narrow that down to 900px or so and see -
There is still plenty of other problems though. All the absolute positioning is really messing things up. Try removing the height:3000px; and you'll see the black background disapear.
margin: 0 auto;
Run your code through the validator and see if fixing as many errors as you can doesn't improve things - see my suggestion and links about validating in my sig below.