Page 77 - CSS

Using box-shadow:

.div2{
border: 5px solid green;
box-shadow: 0px 0px 0px 4px #000;
width: 100px;
height: 100px;
margin: 20px;
}

Using a pseudo element:

.div3 {
position: relative;
border: 5px solid #000;
width: 100px;
height: 100px;
margin: 20px;
}
.div3:before {
content: " ";
position: absolute;
border: 5px solid blue;
z-index: -1;
top: 5px;
left: 5px;
right: 5px;
bottom: 5px;
}

http://jsfiddle.net/MadalinaTn/bvqpcohm/2/

Creating a multi-colored border using border-image

https://riptutorial.com/ 55