You need to edit your style.css file; the colours you want to change are defined in #leftnav, #rightnav and #content, using the background value.
The hex code for black is #000000 (can be abbreviated to #000).
Personally, I wouldn’t change the colour to black as I find white text on a black background very difficult to read on a screen. It tends to strain my eyes and I give up reading. But that’s me; others might feel differently.