Set a button, or dropdown menu, that calls a function to set a cookie and to switch between CSS files which contain the _style
ID:
function switchStyle(style) {
var file = $TMPL_URL + '/css/' + style;
jQuery('head').append('<link rel="stylesheet" href="' + file + '" type="text/css" />');
jQuery.cookie($TMPL_NAME + '_style', style, {
expires: 365,
path: "https://wordpress.stackexchange.com/"
});
}
jQuery.cookie
should be replaced with whatever your standard cookie function is. Change expires: 365
to set whatever duration you wish, so the page remembers which setting was chosen for that user.