The Simple theme has been built using responsive styling. This means when you reduce the width of your browser window, the site will adapt to the screen size, a technique made possible using HTML5.
Responsive web design is an approach aimed at crafting sites to provide an optimal viewing experience - easy reading and navigation with minimal resizing, panning, and scrolling - across a wide range of devices (from mobile phones to desktop computer monitors).
This behaviour is provided by the default with the SilverStripe 3 theme. Alternatively, you can also view this page on your favourite smartphone to see how the theme adjusts to a specific device. The menus and other elements of the page will change to be mobile or tablet friendly.