These are a couple simple tweaks to make an offset background and border inspired by the Mozilla VPN website.

/* Offset Border Box */
.border-offset{
position: relative;
}
.border-offset:before{
content:'';
position: absolute;
width: 100%;
height: 100%;
top: 2em;
left: 2em;
background-color: #666666;
border-radius: 4px;
}
.border-offset:after {
content: '';
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
border: 4px solid black;
border-radius: 8px;
}
For instructions, please see this video tutorial: