penpot/frontend/resources/styles/main/layouts/viewer.scss

.viewer-layout {
  display: grid;
  grid-template-rows: 40px auto;
  grid-template-columns: 1fr;

  &.fullscreen {
    .viewer-header {
      opacity: 0;
      &:hover {
        opacity: 1;
      }
    }

    .viewer-content {
      grid-row: 1 / span 2;
    }
  }

  .viewer-header {
    grid-column: 1 / span 1;
    grid-row: 1 / span 1;
  }

  .viewer-content {
    grid-column: 1 / span 1;
    grid-row: 2 / span 1;
  }
}