The 5 boxes (columns) are not centering because this theme is laid out with a 12-grid column layout. This means you cannot have 5 go into 12 equally. You can only have 1, 2, 3, 4, 6, or 12 boxes because each divides into 12 equally. So for your layout, you will need to lose one of the boxes or add one more for 6. The only other way of doing 5 boxes would be to go totally custom HTML and make your own columns, but then you will need to know how to code it so that on mobile phones, they will adjust accordingly. The other option is a page builder plugin which basically creates widget sidebars (modules) in a blank page (this theme has a page template for a page builder. Using a page builder plugin will allow you to get 5 in there equally laid out.