ad

@media only screen and (max-width: 400px) {
  .box {
    background-color: red;
  }
}

@media only screen and (max-width: 800px) {
  .box {
    background-color: blue;
  }
}

@media only screen and (max-width: 1200px) {
  .box {
    background-color: yellow;
  }
}