Hi!
To disable the responsive design you have to follow the instrucion:
1) open file functions.php in text editor and remove there following code:
wp_enqueue_style( 'mobile', get_template_directory_uri() . '/styles/mobile.css', '', '1.0.2', 'screen and (min-width:240px) and (max-width:639px)' );
wp_enqueue_style( 'tablet', get_template_directory_uri() . '/styles/tablet.css', '', '1.0.2' , 'screen and (min-width:640px) and (max-width:1023px)' );
2) remove all code starting from strings :
/* TABLET */
in the file style.css.