You can create a jQuery function to unload Bootstrap CSS files at the size of 768px, and load it back when resized to lower width. This way you can design a mobile website without touching the desktop version, by using col-xs-* only
function resize() {
if ($(window).width() > 767) {
$('link[rel=stylesheet][href~="bootstrap.min.css"]').prop('disabled', true);
$('link[rel=stylesheet][href~="bootstrap-theme.min.css"]').prop('disabled', true);
}
else {
$('link[rel=stylesheet][href~="bootstrap.min.css"]').prop('disabled', false);
$('link[rel=stylesheet][href~="bootstrap-theme.min.css"]').prop('disabled', false);
}
}
and
$(document).ready(function() {
$(window).resize(resize);
resize();
if ($(window).width() > 767) {
$('link[rel=stylesheet][href~="bootstrap.min.css"]').prop('disabled', true);
$('link[rel=stylesheet][href~="bootstrap-theme.min.css"]').prop('disabled', true);
}
});