Is it possible to change the color of a horizontal line made with "---"?

As the title asks, the horizontal rule makes a white line that goes across. I’d like it to be dark green on all pages I use it on. Any possible way to do that in the stylesheet?

Yep.

According to the documentation, the horizontal rule markup is turned into a hr element.

As you guessed, all you have to do is to apply some css styling to that hr html element.

For example just add this snippet and fine tune it to your needs in the Edit Story Stylesheet menu option…

hr
{
  height: 3px;
  background-color: darkgreen;
  border: none;
}