hello.scss

.btn {
 background-color: #f00;
 display: block;
 
 &:hover,
 &:focus {
  background-color: red; 
 }
 
 &:active {
   background-color: blue;
 }
 
 .js & {
  opacity: 0; 
 }
 
 @include respond-to(40em) {
  display: inline-block; 
 }
 
 @include respond-to(960px) {
  background-color: #000; 
 
   &:hover,
   &:focus {
    background-color: blue; 
   }
   
   &:active {
     background-color: red;
   }
 }
}