according to W3 schools
The padding clears an area around the content (inside the border) of an element. The padding is affected by the background color of the element.
so basically it has no color property to make it an opaque or adding some opacity, I think what you need is 'border' ex.
border: 20px solid #000;
JSfiddle: http://jsfiddle.net/bJ2CF/2/