Massilla

By old and new, do you mean the classic templates stylesheet_original and stylesheet_new?

Just changing everything that is white to black won't necessarily cause the change you're wanting. It's possible that the thing you want to change may have no color, i.e. appear white by default and so not be mentioned in the stylesheet.

If what you want to do is turn the background color for your center column black, then just add the following to your stylesheet_new
.centerColumn, #navBreadCrumb {background-color:black}