I was wondering if there is any way to code the green shadow in that shape with css
You can stack the horizontal and vertical offset values of the box-shadow
property, alternating between two colors and specifying as many layers as you want, e.g.
div {
background: olive;
box-shadow:
0 01px black,
01px 0 green,
01px 02px black,
02px 01px green,
02px 03px black,
03px 02px green,
03px 04px black,
04px 03px green,
04px 05px black,
05px 04px green;
height: 50px;
width: 50px;
}