Graham bemethavinci you normally just add the supplied bootstrap css link to the _headtags
or you can upload the file to your site and add your own link
if you still want the xtgem_template.css file for the xt_touch and 'active' navigation class just add a dummy class to a empty xtgem_template.css file .dummy {}
I have not tried copying the entire bootstrap css to xtgem_template.css
if you want to use xtgem_template or global_stylesheet to add extra classes then remember these files are linked at the top of the header so you may have to add !important to any custom value so it overrides the bootstrap setting
classes declared in global stylesheet will also appear in the building tool class selector