Graham Waplocus Subdomains are both easy and tricky at the same time
see http://syntax.xtgem.com/manual/xtgem/default?func=subdomain

it is far easier to have the same html code and pages with different css using the media attribute of the link rel
http://www.w3schools.com/tags/att_link_media.asp
or
@media queries in the css file
http://www.w3schools.com/cssref/css3_pr_mediaquery.asp

Xtgem actually has a built in method using the xt_touch class that requires you to be using xtgem_template.css as one of your css files to activate it
basically it works like this
.xt_touch .className {...} only applies to pc laptop and high end mobiles tablets etc
.className {...} applies to all devices
When you visit with a high end device the class xt_touch is applied to the body and any other classes can be referenced from that