add background image to just the page area of just one page

I’m am NOT a coder, just a designer, that is starting to get an idea of how .php works, a little… but can write the code my self, yet. I’m using Twenty Twelve and here is what I’m trying to do:

1. I want to be able to target a specific page in my clients WP site with CSS so that it only effects one specific page. At this time we are NOT using a blog.

2. I want to add a background image to the “page” area only (the off white area only, see site: https://sivarods.com/) the overall background is fine, not a problem, since it remains constant throughout the site.

I’ve spent 4 or 5 days searching for the answer… the “.page-id-# ” answer doesn’t seem to change anything. I’m probably missing something. I think if I understood how setting things up so “.page-id-# works or what I have to do to make that kind of editing work, I probably would never need another question answered. I’ve tried many variations of the following CSS:

.page-id-51 #page {
background:000000 url(‘https://sivarods.com/wp-content/uploads/2013/03/lab1.2.jpg’)!important;
}

I also tried a test on the R&D page to just change the background color….

page-id-12 #page.hfeed {
background-color: #cccccc;
}

Didn’t work either.