If you wish to change the display to a specific screen size, a new style sheet can be called. The following code should be placed in the header.php
Calling a WordPress stylesheet smartphone:
<link href="<?php bloginfo( 'stylesheet_directory' ); ?>/mobile.css" rel="stylesheet" type="text/css" media="all and (min-width: 0px) and (max-width: 600px)" />
Calling a WordPress style sheet for tablet
<link href="<?php bloginfo( 'stylesheet_directory' ); ?>/tablet.css" rel="stylesheet" type="text/css" media="all and (min-width: 600px) and (max-width: 800px)" />