For some reason I don't understand, styling .categoryListBoxContents in your stylesheet_books.css works fine, though it doesn't work at all in your main stylesheet, even at the very end of it.

There is no reference to .categoryListBoxContents in stylesheet_books.css to override a reference in stylesheet.css.

Likewise, a basic declaration at the bottom of stylesheet.css like this

#mainWrapper { background-color: blue;}

has no effect, but turns the background blue when put at the top of stylesheet_books.css.

I dunno what's going on...