I tryed to use directly the mainbody/no-sidebar but it didn't work as I want.
I want to have the content block with of 100% and finaly I found in the template.css (around line 2543)
@media screen and (min-width: 992px) {
.items-leading .item-ct {
width: 55%;
}
sdsdsd
I replace the 55% by 100% and it works fine. When you remove sidebar 1 and/or 2 in the layout configuration with the JA template manager, the core content take all the free space on the welcom page.
I just hope I will not have other side effect 🙂