Add style for 4K display

There are several things to do:

A) FONT

html {
     font-family: Arial, Helvetica, sans-serif;
     font-size: 62.5%;
}

@media (min-width: 2000px) {

     html {
          font-size: calc(62.5% * 1.5);
     }

} 

@media (min-width: 3000px) {

     html {
          font-size: calc(62.5% * 2);
     }

}

@media (max-width: 768px) {

     html {
          font-size: calc(62.5% * 0.8);
     }

}

B) Brakponit "em" unit

// BREAKPOINTS
$screen-xs-min: 30em;
$screen-sm-min: 48em;
$screen-md-min: 64em;
$screen-lg-min: 82em;

C) Scales by HTML elemnt in rem unit

10rem = 10px