index.html

<div class="container">
  <aside class="toolbar">.toolbar</aside>
  <section class="preview">.preview</section>
  <section class="timeline">.timeline</section>
</div>

styles.css

* { box-sizing: border-box; }

body {
  margin: 0;
}

.container {
  display: grid;
  width: 100vw;
  height: 100vh;
  grid-template-columns: 200px auto;
  grid-template-rows: auto 130px;
  grid-template-areas: 
    "toolbar editor"
    "timeline timeline";
}

.nav {
  grid-area: header;
}

.toolbar {
  background: orange;
}

.preview {
  background: gray;
}

.timeline {
  background: green;
  grid-area: timeline;
}