if(navigator.userAgent.toString().indexOf("WebKit") != -1)
{
    document.write('<link rel="stylesheet" type="text/css" href="' + 
        base_path + 'style/webkit.css" />');
}
else if(navigator.userAgent.toString().indexOf("Gecko") != -1)
{
    document.write('<link rel="stylesheet" type="text/css" href="' + 
        base_path + 'style/gecko.css" />');
}