It is working now. Resuming the steps and correct me if I wrong. Just in case somebody else is reading:
** Creating background for specific article. (In this case the article is selected as a menu-item)
- Go to menu manager
- Select menu
- Go to menu item you want for editing. Click open or edit
- Under Page Displays Options add in CSS box your own class: " mystyle" (space before the word)
- Save it
NOTE: If you inspect the element (firebug) the div displays like:
<div clas="item-page mystyle">
IGNORE item-page
- Go to your CSS file
Optional add note
- Add your style
.mystyle
{
background-color: #000000;
} Thanks a lot for your help.