/* ensure body fills the viewport height */
body {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
  margin: 0;
}

/* this is your full-width colored band */
.thank-you-section {
  flex: 1;                      /* fill between header & footer */
  position: relative;
  left: 50%;                    /* break out of any container centering */
  right: 50%;
  margin-left: -50vw;
  margin-right: -50vw;
  width: 100vw;                 /* full viewport width */
  background: var(--highlight);
  
  display: flex;
  justify-content: center;      /* center the inner container */
  align-items: center;          /* vertically center it too */
  padding: 2rem 0;              /* vertical breathing room */
}

/* keep your text at a readable width */
.thank-you-main {
  text-align: center;
}