Changing background of index center column

[Decostyle]

Hello everyone,

I want to change the background of the centercolumn index categories page only (mainpage) where pictures appear per top category.

The stylesheet css I used is:
.centerColumn #indexCategories {
background-color=#dfdfdf;
}

For some reason this does not work. if I remove the #indexCategories then it works, but it adds background color to all the centercolumn pages. I only want it to apper on the main page indexCategory.

Any idea how to do this?