I think you can move this `background-position: center top` to https://github.com/mozilla-b2g/gaia/blob/master/apps/communications/ftu/css/style.css#L628 .
For getting the complete UI you need to modify https://github.com/mozilla-b2g/gaia/blob/master/apps/communications/ftu/css/style.css#L632 and change to 100%. Thanks!