In short, yes it can be done entirely with CSS, upload just the zca_responsive.css and zca_responsive_default.css files, merge the html_header.php (without the $_SESSION['layoutType'] variables) and tpl_main_page.php file and your good to go!
However, what will be unnecessary is all the CSS media queries added for tablet and desktop views while visitors visit the site via a mobile device.
The zca_responsive_default.css loads on the desktop and "hides and manipulates" content for all views whereas the zca_responsive_mobile.css only get loaded on mobile devices and only contains the CSS changes for mobile devices.
The $_SESSION['layoutType'] variables allow you to include jQuery for mobile devices only and not tablet and desktops and vise versus.
The $_SESSION['layoutType'] variables allow to swap out a "desktop header" file for a "mobile header" with less code to load.
The $_SESSION['layoutType'] variables allow to add tabs to the "desktop" product info page and a mobile friendly accordion to the "mobile" product info page without the need to "hide or manipulate".
The code is "open source" and you can use it, manipulate it, change or alter it in any way you see fit. Good or bad, right or wrong!