I believe just the following should do what you want:
background:#F9F0C9 url('file:///C:/Program Files (x86)/Calibre2/resources/images/old_paper.jpg');
If the book you load includes it's own background color (the #F.. portion) and image (the url portion) it might override the info above. Adding " !important" (including space but without quotes) after the closing parenthesis and before the semi-colon might correct this issue. (I've never used an image. It will definitely override the color.)
CSS, kind of like missing ebook metadata, doesn't complain if various bits and pieces are not present. CSS just assumes that the defaults apply and only the mentioned bits are being changed.
The body section affects the page itself; the p is for defining paragraphs, the h1-p6 are various heading definitions. The selection... is beyond my knowledge.