What do I need to do to add | to styles, so I can define my own styles?

As you can see in this example: https://www.reddit.com/r/homeassistant/comments/cj0s0e/want_to_share_my_whats_next_on_prime_and/
the person has a pipe (|) next to the style “attribute”. The person can then add basically whatever styles they want.

If the plugin is called card-mod, then please tell me why it didn’t work. All the styles I applied, were added with 1 character per CSS style. Basically this:

.element {
  m;
  a;
  r;
  g;
  i;
  n;
}

for margin and so on. Invalid CSS. My yaml looked like this:

  type: custom:list-card
  entity: sensor.news
  title: News
  feed_attribute: entries
  columns:
	- title: ""
	  field: text
	  style: |
		tbody tr:not(:first) {
		  border-top: 1px solid white
		}

As you can see, I need to add some CSS styling to every element except the first. But I can’t do that with the build in styles, like this:

style:
  - border-top: 1px solid white