There is a special CSS class that controls the background color of the decorated zone or respectively to the entire page with RadFormDecorator - it is RadForm_SkiName.rfdZone
You may use that selector to set the fore or back color that you like, and actually to add any custom properties and values to it.
The case with the fieldsets is slightly different - to style its background we actually use a large (2000 x 2000 pixels) non-repeatable image. Unfortunately, due to poor background color support for fieldsets in IE, combined with the rounded corners we actually had to stick to background image, rather than to normal color values. In order to change the background of a fieldset, you need to actually create a new background image and apply it, as we have done in our skins, i.e:
border: solid 1px #080808;
Have a great weekend,
the Telerik team