.box {
   max-width: 1000px;
   margin: 50px auto;
   display: grid;
   grid-gap: 10px;
   grid-template-columns: 250px;
}

 header, main {
    border: 2px dotted black;
}

header {
  grid-row: 1 / 2;
  grid-column: 1 / 3;
}

main {
  grid-row: 2 / 3;
  grid-column: 3 / 4;
  height: max-content;
}

header {
    height: auto;
    background-color: #292929;
}

main {
    height: auto;
    background-color: #292929;
}