XF 1.4 Solved - mainContainer_noSidebar , where can I find it in Style Properties?

Hello there,
I am wondering where I can find mainContainer_noSidebar in Style Properties, I know I can edit it thought Templates but I rather do it there.

Thanks in advance.
This is a useful site for learning the basics of CSS and HTML: http://www.w3schools.com/css/

You just need to edit the classes to add the styling you want.
In EXTRA.css I try to do it like this from the link you showed me:
.mainContainer, .mainContent, .mainContainer_noSidebar, .WidgetFramework_WidgetRenderer_ShareThisPage{
    margin-bottom: 15px;
But thats add a 15px gray space below the .mainContainer_noSidebar block, I have a background image. Any Any alternative way of doing this?
I also tried bottom and padding-bottom but they didn't seem to do anything.