1. How to set individual background to any post? Plugins like custom background, background per page,...and a like, does not work...
Every post, page, widget, etc. in WordPress gets a unique ID. If you use your browser's "View Source" function, or the Firebug addon for Firefox, it is very easy to find that ID and target that particular page with CSS. No plugin necessary. There are many examples of this in this forum.
I have tried editing style.css on content area and nothing...like it does not work
First, don't edit theme files, or you will lose your changes at the next update. Second, be sure to flush your browser cache (and your server cache, if you are using one) after making changes.
they didn't say anything where to put code exactly or alternative way
There are two ways to do this. First way: add your CSS code to the Graphene Custom CSS option (under Graphene Options >> Display Options). Second way: create a child theme and add your CSS code to your child theme stylesheet.
Why does plugins that are made for this don't work?
There's no point in using a plugin for something that you can control with CSS. The more plugins that you use, the greater your chance of having a plugin conflict.
Without knowing which plugins you have tried, it's impossible to say exactly why they didn't work, although caching is always an issue. (See above.) It's possible that they were written for a theme or theme that puts its posts and pages together differently.
We can't really teach basic CSS in this forum (there are resources listed here), but we can definitely point you in the right direction if we have some idea of how much CSS you already know. How good are you with basic HTML and CSS?