<div></div>

body {
  background: gray;
}
div {
  height: 400px;
  width: 400px;
  margin: 0 auto;
}

div {
  border-image: conic-gradient(red, yellow, lime, aqua, blue, magenta, red);
  border-image-slice: 1;
  border-width: 4px;
  border-style: solid;
}