.stack {
  border-left: 3px solid #4CAF50; /* green (level 1) */
  margin-left: 10px;
  padding-left: 10px;
  font-family: monospace;
}

/* level 2 */
.stack .stack {
  border-left-color: #2196F3; /* blue */
}

/* level 3 */
.stack .stack .stack {
  border-left-color: #4CAF50; /* back to green */
}

/* level 4 */
.stack .stack .stack .stack {
  border-left-color: #2196F3; /* back to blue */
}