<div class='box'></div>

.box {
  position: relative;
  height: 100px;
  width: 100px;
  background: white;
  $border: 1px;

  &::before {
    content: "";
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    margin: -$border;
    z-index: -1;
    background: linear-gradient(to right, red, orange);
  }
}

html{
  background: lightgray;
}