@property --background-angle {
  syntax: "<angle>";
  inherits: true;
  initial-value: 0deg;
}
@property --color {
  syntax: "<color>";
  inherits: true;
  initial-value: transparent;
}
.container {
  width: 20rem;
  aspect-ratio: 3/4;
  --background-angle: 45deg;
  --color: #e976e9;
  background: linear-gradient(var(--background-angle), var(--color), #57c582);
  -webkit-animation: color-swing 1.5s ease infinite;
          animation: color-swing 1.5s ease infinite;
  position: relative;
  border-radius: 0.2rem;
  box-shadow: 0 0 2rem 0.5rem rgba(0, 0, 0, 0.5);
}
.container:before {
  --size: 2rem;
  content: "";
  position: absolute;
  background: var(--labs-sys-color-background);
  top: var(--size);
  right: var(--size);
  bottom: var(--size);
  left: var(--size);
  border-radius: inherit;
  box-shadow: inset 0 0 2rem rgba(0, 0, 0, 0.9);
}

@-webkit-keyframes color-swing {
  50% {
    --background-angle: 145deg;
    --color: #75e4ee;
  }
  to {
    --background-angle: 45deg;
    --color: #e976e9;
  }
}

@keyframes color-swing {
  50% {
    --background-angle: 145deg;
    --color: #75e4ee;
  }
  to {
    --background-angle: 45deg;
    --color: #e976e9;
  }
}
body {
  display: grid;
  place-items: center;
  min-height: 100vh;
  background: var(--labs-sys-color-background);
}

:root {
  --labs-sys-color-background: #222;
}