Any Image Set in Background Manager has the option to include its own additional CSS styling. This can be used to create more complex overrides.
Let’s say you have a page dedicated to oranges, like this page. Not only will the background contain an image of oranges, but the theme colors are matched to the background.
But doesn’t have to be just colors. For example, a shadow has been added to the main content of this page (visible in CSS3 compatible browsers), a background image has been added to the content and the text formatting has been changed.
This was all possible using the optional CSS styling for the Image Set, without editing any files directly.
( Vintage paper texture by pinkorchid_too (Sandra) )