The box-shadow CSS property adds shadow effects around an element's frame. You can set multiple effects separated by commas.

A box shadow is described by X and Y offsets relative to the element, blur and spread radius, and color.

Tutorial.

HTML - Markup

<div class="box-container">

<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>

</div>

CSS - Styles

Syntax

/*Syntax*/
box-shadow: horizontal-offset | vertical-offset | blur-radius | Color;

/*Example with values*/
box-shadow: 10px 5px 5px black;

The horizontal offset (required) of the shadow, positive means the shadow will be on the right of the box, a negative offset will put the shadow on the left of the box.

The vertical offset (required) of the shadow, a negative one means the box-shadow will be above the box, a positive one means the shadow will be below the box.

The blur radius (required), if set to 0 the shadow will be sharp, the higher the number, the more blurred it will be, and the further out the shadow will extend. For instance a shadow with 4px of horizontal offset that also has a 4px blur radius will be 8px of total shadow.

Color (required) – This takes any color value, like hex, named, rgba or hsla. If the color value is omitted, box shadows are drawn in the foreground color (text color). But be aware, older WebKit browsers (pre Chrome 20 and Safari 6) ignore the rule when color is omitted.

/*Layout style*/
body{
display: flex;
flex-direction: column;
align-items: center;
justify-content: center;
}

.box-container{
display: flex;
flex-wrap: wrap;
}

/*Layout styles end here*/


/*Box & shadow styles below*/
.box{
height:180px;
width:180px;
background-color:#00796b;
border:2px solid #808080;
margin: 0 0 20px 20px;

box-shadow: inset 2px 5px 5px rgba(0,0,0,.55),2px 5px 5px rgba(0,128,0,.55);
}

To learn more about this css box shadow property visit Developer.mozilla.org

You can also create box shadows easily using this non-profit project, made by developers for developers: CSS Matic Site