You cannot use CSS code in the OPF.
If "cover.jpg" is used in some XHTML file, in a <img> element, then there's where you have to add/modify the CSS (have a look at some tutorial on HTML/CSS to know what you are doing).
If "cover.jpg" is only referred in that guide element, then I don't think there's nothing you can do, and the reader is just buggy.