when size of screen is mobile css code example

Example: css media query

/* BOOSTRAP MEDIA BREAKPOINTS */

/* Small devices (landscape phones, 576px and up) */
@media (min-width: 576px) {  
  .selector {
  	background-color:#f00;
  }
}
 
/* Medium devices (tablets, 768px and up) The navbar toggle appears at this breakpoint */
@media (min-width: 768px) {  
}
 
/* Large devices (desktops, 992px and up) */
@media (min-width: 992px) { 
}
 
/* Extra large devices (large desktops, 1200px and up) */
@media (min-width: 1200px) {    
}