create css style for diffrent screen size by javascript code example

Example 1: media query

@media only screen and (max-width: 600px) {
  body {
    background-color: lightblue;
  }
}

Example 2: minimum width of different devices in html

body {
  background: white;    
}

@media screen and (min-width: 980px) /* Desktop */ {
  body {
    background: red;
  }
}

@media screen  and (max-width: 979px) /* Tablet */ {
  body {
    background: blue;
  }
}

@media screen and (max-width: 500px) /* Mobile */ {
  body {
    background: green;
  }
}