如何向包装器内部的div(具有id)添加背景色?

Below is my code. Im using the framework skeleton. I have separate divs that represents three,seven and six columns that equal 16(skeleton is a 16 column framework). Therefore the the 3 divs are all inline accross the screen. The wrapper has a background color grey, therefore making the whole scree grey. However, i want to make the background color of the 3 div classes black. Any ideas? As ive tried to put all 3 divs into another div with an id, and set the background black however did not work? Below is my code. Im using the framework skelet