Box shadow declarations can confuse me. So I decided to create a jsfiddle to show the basics and where I could play with additional styling in the future.
You can use box-shadow to help create double borders, and you can play with the values to get just shadows on certain sides of the box. You can shadow inside or outside the box. You can add multiple values for box-shadow, separated by commas, to build up your styling.
Here is a great reference for box-shadow, with additional examples.