layout-sidebars.scss

index.html

<header><h1 contenteditable>Header.com</h1></header>
<div class="left-sidebar" contenteditable>Left Sidebar</div>
<main contenteditable></main>
<div class="right-sidebar" contenteditable>Right Sidebar</div>
<footer contenteditable>Footer Content — Header.com 2020</footer>

layout-sidebars.scss

body {
  display: grid;
  height: 100vh;
  grid-template: auto 1fr auto / auto 1fr auto
}

// etc

header {
  background: lightpink;
  padding: 2rem;
  grid-column: 1 / 4;
}

.left-sidebar {
  background: lightblue;
  grid-column: 1 / 2;
}

main {
  background: coral;
  grid-column: 2 / 3;
}

.right-sidebar {
  background: yellow;
  grid-column: 3 / 4;
}

footer {
  background: wheat;
  padding: 2rem;
  text-align: center;
  grid-column: 1 / 4;
}