body { margin: 0; }
.drop {
  --drop-margin: 1ch;
  width: calc(100vw - calc(var(--drop-margin) * 2));
  height: calc(100vh - calc(var(--drop-margin) * 2));
  background: #222;
  outline: 4px dashed grey;
  margin: var(--drop-margin);
}
.drop:after {
  content: 'Drag and Drop Video';
  color: white;
  margin: calc(50% - 10ch);
  font-family: monospace;
}