As I explained when I pointed this out, the reason it doesn't affect the page background is that the selector:
appears nowhere in the markup of the page.
Because of that, I suggested that the entire section:
could be removed.
/* #ffff00 = yellow */
border:8.0px solid blue;
The only reasons for updating it to standards and keeping it would be if it's also used on another page with a class="intro" element, or if there's a possibility that a class="intro" element will be added to the page later.
As further explanation, the layer-background-color property was required for very early Netscape browsers. No browser that requires that is in use today. If it were, it would have many more problems in rendering the typical modern web page than getting the background color right.