$size: 9em;
$strip: .25em;

html {
  background: 
    linear-gradient(90deg, 
        #fff $strip, transparent 0), 
    linear-gradient(90deg, 
        #36599d 50%, transparent 0), 
    linear-gradient(90deg, 
        #643140 50%, #0a226a 0);
  background-size: 
    .25*$size, .5*$size, $size;
}