The layouts for the mainbody area is slightly different from the other widget layouts. The primary difference is that the mainbody is generally displayed along with up to 3 sidebars. This provides the ability for Myriad theme to effectively support a four-column layout.
Configure at Admin Dashboard → Myriad Theme, then go to Layouts tab and set the varying Mainbody/Sidebar layout possibilities.
Note: If no widgets are placed in the Sidebar position, the Mainbody will become full width.