Setting Background of only certain pages with Custom CSS

I am not super CSS literate but I’m working on learning.

On the webpage in question I only want two pages (And their children) to have a darker background from the rest of the website. I thought I had it figured out with page-id-119 but that didn’t seem to work, or I’m putting it in the wrong location in the code.

Can anyone tell me where in the following code I need to put the page id for it to work correctly? And is it possible to extend it to all child pages or will I have to repeat this code with the id of every child page?

.panel > .panel-body {
  background-color: #000000;
}
.main-content {
  background-color: #1a1a1a;
  color: #ffffff;
}
.main-content h3 {
  color: #ffffff;
}