@media not all {
  div {
    color: blue;
  }
}

@media all {
  /* Nested rule */
  @media (max-width: 400px) {
    div {
      color: red;
    }
  }
}

div {
  width: 20px;
  height: 20px;
  background-color: ghostwhite;
}

@media (max-width: 400px) {
  div {
    color: green;
  }
}