CSS: use "background" shorthand property

CSS: use "background" shorthand property

It's more compact and works just as well.  In fact, I think it might
work better on browsers that support background-image but not
background-size.  On such browsers the entire rule will be ignored,
which is what I want.  If the background image can't appear at
precisely the correct size, it shouldn't appear at all.

Of course, ideally on a browser that fails to display the background,
the title also wouldn't fade out to the left.  I wonder if there's a
sensible way to achieve that.