#editor {
  flex: 10;
}

.lineMarker {
  position: absolute;
}

.lineMarker::before {
  content: "";
  position: absolute;
  opacity: 0.5;
  width: 100%;
  height: 100%;
  left: 0;
  z-index: 5;
}

.lineMarker.red::before {
  background: darkred;
}

.lineMarker.grey::before {
  background: grey;
}