Graham bemethavinci You would almost need to write your own version of bootstrap or one of the other templates
A lot of the 'responsive parts of bootstrap are based on @media css queries plus javascript/jquery for the drop menus and some other stuff it would take a lot of work and study possibly years of work to create your own version
Xtgem does have a couple of built in tools like the xt_touch class that is applied to the body tag for high end mobiles/tablets and pc's (if you are using the xtgem_template.css file to hold all or part of your css) and is used in the xtgem css templates there is also the <xt:get_device_template> function that returns either 'web' or 'mobile' and the data.device variable in the blogs twig data array that returns web/touch/mobile
personally I cannot see any real benefit in redoing work that is covered in bootstrap or any of the other css template even major sites use them
To do it you would need to study everything to a high standard html css javascript jquery possibly some ajax and you would really need a pc or at least a good tablet to do it on
you are far better of spending your time learning how to use bootstrap or one of the other templates and concentrating on the content of your site