SOURCE

<link rel="stylesheet" href="style.css" media="only screen and (max-width:320px)" />

@media (max-width:320px) {
  body{
    background:red;
  } 
}
@media (min-width:321px) and (max-width:375px) {
  body{
    background:blue;
  } 
}
@media (min-width:376px) and (max-width:425px) {
  body{
    background:green;
  } 
}
@media (min-width:426px) {
  body{
    background:yellow;
  } 
}