var link = [];
link[0] = "http://site.com/css/style1.css";
link[1] = "http://site.com/css/style2.css";
link[2] = "http://site.com/css/style3.css";


$(function() {
    var style = link[Math.floor(Math.random() * link.length )];
    $('<link />',{
        rel :'stylesheet',type:'text/css',href: style
    }).appendTo('head');
});

编辑：谢谢Basil Siddiqui！

var link = [];
link[0] = "http://site.com/css/style1.css";
link[1] = "http://site.com/css/style2.css";
link[2] = "http://site.com/css/style3.css";


$(function() {
    var style = link[Math.floor(Math.random() * link.length )];
    if (document.createStyleSheet){
        document.createStyleSheet(style);
    }else{
        $('<link />',{
            rel :'stylesheet',href: style
        }).appendTo('head');
    }
});