<header class="header">Header</header>
<main>Main</main>
<footer>Footer</footer>

html,
body {
  margin: 0;
  padding: 0;
}
body {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
}
main {
  flex: 1 0 0;
}
header,
footer,
main {
  padding: 10px;
  background: cyan;
  border: 1px solid red;
}
main {
  background: aquamarine;
}