I'll add this about Kubrick: virtually all background elements (including the content area) you see on the screen when viewing a blog with Kubrick is an image. The background for the main content area is actually three different images, and because of the drop shadow around the content area, simply changing the grey background image (around the outside) leaves you with a grey gradient border around the content area.
The above is the long way of saying, it isn't just a matter of giving you the ability to change the background, or the background color. It takes a huge amount of work to make it so that you would be able to do that from within the dashboard, and have an acceptable result. Even the original designer knew that, and instead of updating that theme, the designer came out with K2 Lite, which is the replacement for Kubrick.
Here is an example of what I mean: http://opposablethumbz.files.wordpress.com/2010/05/kubrick.png .
As you can see, then entire background for the content are would have to be created anew before they could give you the ability to change the background from within the dashboard, and any change they did, as slikbonez says, would upset those that like it the way it is.
Kubrick, from a design standpoint is difficult to work with.