This page does not have any ID and that's why it does not work. You used page ID 79 what is wrong for that page. This is under construction page. If you don't know how to check classes for elements, we recommend to have a look into documentation where we explained how it works.
The url you sent to screenshot, does not work. It's empty. But of course, now you sent url to the right page. ID for this page is 79 of course as you mentioned. As we see, the css we gave you does not work but we are a little bit confused now because we don't know which color you want to change exactly. Do you want to change color of section which is currently black o grey? Which one you want to control with css?
So this is unfortunately not possible. We thought that you mean content section. The part of website you mentioned can be setup only one per page because page id is below the html in hierarchy. The section you showed is as we call it "Nothing" because you don't have any content on page. If you will do any content for page, this section will be invisible.
Sorry but your English is very hard to understand. From current messages, we have no idea what you mean.
As we wrote above, you got this white space because you don't have content at all and this color can be changed only once (the same for all pages). But it is not possible to change this color per pages. Per pages it is possible to change content background only.
URL does not work again. Please send us working url. Also will be great if you can tell us how you did it because we wonder what you did. In our opinion it is not possible because html is higher in hierarchy of html code then page id.
Comments
to change background colors per different pages, you must use below css: where xxx is page id.
http://prntscr.com/74rdaf
Now you are talking about CSS Class? CSS class and product ID are two different things.
Maintaining this off so you can see better.
This is the link of the page. http://bit.ly/1E5vrZV
Thank you.
As we wrote above, you got this white space because you don't have content at all and this color can be changed only once (the same for all pages). But it is not possible to change this color per pages. Per pages it is possible to change content background only.