Edit in JSFiddle

<header class="o-header">Header</header>
<aside class="o-aside">Aside</aside>
<main class="o-main">Main</main>
<footer class="o-footer">Footer</footer>

body {
  display: grid;
  grid-gap: 1rem;
  grid-template-areas:
    "header header header"
    "aside main main"
    "footer footer footer";
  grid-template-columns: auto auto auto;
  grid-template-rows: auto 100vh auto;
}

.o-header {
  grid-area: header;
}

.o-aside {
  grid-area: aside;
}

.o-main {
  grid-area: main;
}

.o-footer {
  grid-area: footer;
}

.o-header, .o-aside, .o-main, .o-footer {
  background: #BC20E2;
  color: #FDFDFD;
}