css variables

cssvar.css

$a: 4;
$b: 3;

.slide {
   position: absolute;
   top: 0; 
   left: calc(50vw - #{$a/$b/2*100vh});
   width: $a/$b*100vh; 
   height: 100vh;
  
   @media (max-aspect-ratio: #{$a}/#{$b}) {
      top: calc(50vh - #{$b/$a/2*100vw}); 
      left: 0;
      width: 100vw; 
      height: $b/$a*100vw;
   }
}