How can I make a vertical line full height?

Hi, how can I make a vertical line stretch the full height of the code frame? I have the following CSS:

.vertical-line {
  height: 800px;
  width: 2px;
  background: #eeeeee;
  margin-right: auto;
  margin-left: auto;
}

I've tried changing 'height' to 'auto' and '100%' but the line just disappears.

https://www.oppidaneducation.com/families/ucas-journey#how-it-works