.box {
  border: 3px solid #0000;
  border-radius: 5px;
  background: linear-gradient(#131219, #131219) padding-box, linear-gradient(
        var(--angle),
        #070707,
        #687aff
      ) border-box;
  animation: 8s rotate linear infinite;
}

@keyframes rotate {
  to {
    --angle: 360deg;
  }
}

@property --angle {
  syntax: "<angle>";
  initial-value: 0deg;
  inherits: false;
}