❤️ calc, it makes everything so easy:

.thing {
  background-color: #000;

  $maxWidth: 74rem;

  @media ( min-width: $maxWidth ) {
    // This is `max-width: $maxWidth` with a full width background.
    padding-left: calc( 50vw - #{$maxWidth/2} ); // ( (100vw - $maxWidth) / 2 )
    padding-right: calc( 50vw - #{$maxWidth/2} ); // ( (100vw - $maxWidth) / 2 )
  }
}