ID Selector

The ID selector allows you to style an element with a hash tag followed by a given or custom name (#header)

. . .
  #header {
    display: block;
    width: 100%;
  }

  #main {
    display: grid;
    grid-template-columns: 100%;
    grid-template-rows: auto;
  }

  #footer {
    display: flex;
    border: 1p solid #FF9900;
  }