The Xi theme is the ThemeLoom‘s first theme to adopt the Responsive appraoch to design. Not all your users have the same sized screen or the same browser. So this theme will look different on every device and this is by design. It adpats and responds to the size and capabilities of your browser.
You can simulate the effect by resizing your browser window.
Go really small, say 320px wide to see what it looks like on a phone.