@media (max-width: 576px) {  
  .xs {color:red;font-weight:bold;}
}

/* Small devices (landscape phones, 576px and up) */
@media (min-width: 576px) and (max-width:768px) {  
  .sm {color:red;font-weight:bold;}
}
 
/* Medium devices (tablets, 768px and up) The navbar toggle appears at this breakpoint */
@media (min-width: 768px) and (max-width:992px) {  
 .md {color:red;font-weight:bold;}
}
 
/* Large devices (desktops, 992px and up) */
@media (min-width: 992px) and (max-width:1200px) { 
 .lg {color:red;font-weight:bold;}
}
 
/* Extra large devices (large desktops, 1200px and up) */
@media (min-width: 1200px) {  
    .xl {color:red;font-weight:bold;}
}